Why some preamps weigh 50+ lbs?

@mofojo 

Better preamps have the following which adds weight-

Chassis power supply and circuit noise isolation cages or panels.

Perhaps a low noise two chassis design. 

Large, heavier power supply transformers, perhaps two if dual mono. 

Abundant supply of power supply filtering capacitors.

Abundant  supply of signal storage capacitors. 

Heat sinks.

Cabling for multiple switches and connections in and out, balanced and single ended. 

Sheetmetal for a robust sturdy chassis.  

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.