Why some preamps weigh 50+ lbs?

Marketing.

Why do some preamps weigh as light as a feather? Cost cutting, poor parts/design and build quality results in severely compromised sound quality. 

I guess my Benchmark products don’t qualify as high-end then. Preamp with HPA weighing in at 8 lbs ...

Very humble but BM LA4 / HPA4 has very low microphonics due to the fact that its PCB is physically decoupled from the metal enclosure.  Exactly high-end stuffs.

“Why do some preamps weigh as light as a feather?”

SMPS, chip volume control, light faceplate/chassis are all things that could reduce weight.  The power supply is where most of the weight is - transformers, capacitors, and IEC filters.

My Aric Audio Motherlode XL is a 2 box design (separate power supply), and weighs in at a total of 61 pounds for the pair. It also happens to be the best preamplifier I've ever heard.

The Don Sachs/Lynn Olson Raven preamp is brilliant as well, and it's one box design weighs in at just over 30 pounds.

High quality cabinets, shielding, transformers, caps, etc. weigh more