Why outlet makers warn against aluminum wires?

I am looking for an answer as to why there are always warnings on high-end outlets regarding using them with aluminum wires?I use mine (Shunyata Venom) with al.wires and felt the quality jump right away compared to PS Audio i used before.Both outlets manuals say:Do not use with aluminum wires!Why?Thank You for your responses.
Hope this helps
BTW I was UL tester during this time of my life and I remember the stir that AL and AL clad wire caused.
Two issues: Aluminum is more likely to oxidize (corrode or rust) when in contact with certain materials like certain other metals. The oxidation presents increasing resistance at the contact points which causes heating and can lead to fires. Studies have shown aluminum wiring in houses is statistically more likely to lead to fires than copper.

Secondly, aluminum tends to expand and contract more than other materials with changes in temperature, which can lead to loosening of connections over time. To prevent this, either use aluminum-to-aluminum connections or fasteners that resist this type of loosening.

Both of these issues are potential problems that become concerns as time passes. We should weigh them against any sonic benefits that may be audible immediately.
IIRC, aluminum in-wall wiring has been implicated in fires due to oxidation of wires where they attach to outlets. I would GUESS that this is a cya statement
Our house burned down in 1984. Probable cause: Aluminum wiring.
It's more than just oxidation and "cya". Aluminum and copper wire connections can cause arcing which can lead to fire; they need a special crimped connector, which completely encases the interface, to safely join them. The manufacturers of outlets that warn against aluminum do so because the contact between aluminum and the outlet copper straps is not as secure as the connector, and therefore can potentially cause the same arcing to fire problem.
The explanations that Jameswei gives above are correct. After the aluminum wire loosens then it can begin to arc to whichever connection is has come loose from, or even to an adjacent piece of metal. I own a remodeling/home theater business in Boulder, and we have found outlet boxes that are completely black on the inside from arcing over the years. Very dangerous. Aluminum wires should have never been allowed for 110V internal wiring in houses.

A safe, code-approved way to correct the problem is to buy Ideal brand purple wire nuts. Most Home Depots or electrical supply houses carry them. Wire nuts are the rounded-cone plastic pieces, threaded on the inside, that you will see twisted around the ends of electric wires inside of a wall box, where one or more strands of wire are twisted together; the wire nut basically keeping the bare part of the wires squeezed together and isolated from other wires or other metal. All wire nuts are color-coded for size of wire(s) to be inserted. The purple wire nuts have a gooey substance inside that prevents oxidation between the aluminum and copper wires when they are twisted together. A small length of copper wire is twisted onto the end of the aluminum wire, then that twist is inserted into the purple wire nut. The other end of the copper wire is then connected to the outlet or switch/dimmer, etc. You can do this yourself, although I would not recommend it unless you are experienced/ comfortable with doing electrical work. When we do an older home remodel we do the above operation on anything we touch because of the liability issues, and quite often the homeowner pays extra to change out most or all of the other outlets/switches in the house. Licensed electricians usually charge $18-$25 for each device/outlet, incl. the wire nuts. If you have one come out just to change out a few outlets they may want to charge more to cover trip fees, etc.

That would be a great justification for having them run some dedicated 20 amp circuits for your gear outlets while they are there. We now insist on 3 or more such circuits on every theater that we build. If you want more info on that, there have been a lot of posts on that subject here on Audiogon over the last few years.

Good luck.
Fires caused by AL wire. It was banned from being used in homes in canada years ago.
When multiple dedicated circuits are used for audio/home theater is it necessary to run all of the 110V circuits off the same side of the 220V feed?
I have copper wires going from the box into the wall but the ends coming out of the wall are for some reason dull silver colored.That is why i assumed they're aluminum.Can it be some other material?How can i find out?Thanks.
overhang - when was your house built?
>>"I have copper wires going from the box into the wall but the ends coming out of the wall are for some reason dull silver colored.That is why i assumed they're aluminum.Can it be some other material?How can i find out?Thanks."<<

If your home was built in the 50s, or ealier, it is possible the conductors are tinned copper.

With the circuit turned off you can take a knife and scrape the tinned coating off exposing the copper conductor.

Some homes built in the middle to late 70s were wired with AL romex.
I believe the plastic cone wire nuts you reffering to are the MARRETTES as they called in the trade.
Aluminum wire will vibrate with the frequency of the signwave. In this case, 60hz. This causes it to loosen from any connections other than solid crips. When it loosens, you get arcing at the connection. It's also is soft so it will loosen up over time. You need to retighten at least every year.
Al wire is not compatible with copper and cannot be connected together without special al/cu connectors. They do make a grease for this also.
Al wire must be sized larger than copper for current carrying capacity (usually one size bigger eg; #10 copper you would use #8 al.)
A lot of trailers use al and older homes. They used a special al/cu outlet for this purpose. Inserting copper blades from the plug has no effect. You can plug most any connector wire into an outlet with no problem. Just because the conductors are al doesn't mean the plug connector is. It is usually copper(or an alloy.)
Here is a little reading material regarding AL terminations and AL to copper terminations.
Lit is a little old, but still to the point.

The building where i live is a prewar one.
Which war? Korean, Vietnam or Terror (Iraq)?
War of the Worlds.