Why some preamps weigh 50+ lbs?

Most of the weight is typically in the power supply but can also be influenced by chassis damping choices employed by the designer.  The use of transformers can also add weight.

My Esoteric C-03 checks in at 22kg or 48 pounds.

@thecarpathian has it nailed!  The extra weight is there to justify the asking price.

We offer a damping package as an option in two of our preamps. It adds considerable weight. However its effect is both measurable and audible (the latter if the preamp is in the room with the speakers). 

Tube preamps use tubes :) Tubes are subject to microphonic effects. The more vibration they receive the greater the microphonics. Microphonics is a distortion- its added sound to the signal. By damping, by have a more robust chassis, by isomerically isolating sensitive parts you can reduce microphonic effects and thus have a better sounding?lower distortion preamp.  

And that's just one reason...

Then you have passive pre-amps….