Congressional net worth is not up nearly 3700%. It is not claimed in the video you link to and it is not claimed by the Center for Responsive Politics.
Indeed this chart from the CPR suggests that average net wealth in the Senate has been fairly stable and in the House has risen slightly.
The video suggests (around 1:15) 223 out of 535 members of Congress saw their net wealth increase, suggesting more than half did not.
So where does the 3700% number come from? It seems to be this list where one particular congressman saw his net worth increase from $47,503 to $1,790,508. That is partly caused by his low starting point but he still has less than the average by some way. Some others saw their net worth go from positive to negative, but that is not so outrageous.
So the answer is that the video and your headline question are being selective. Some members of Congress get richer, some get poorer. Concentrate on the former and you get something to shout about.
A different issue is that on average members of Congress are much richer than the rest of the population. For some, this might raise questions about how representative they are; for other that is irrelevant in a democracy where voters can choose.