I'll tell you why not. Just like many have said before on this site...DEBT DEBT DEBT! It is common for the U.S. to have recessions or some kind of crisis every five to 10 years (economic cycle or government screw up, you choose)...the debt will exacerbate the "BUST" into something just as bad as 2008 or worse. This bull market is going on 10 years next year...one of our longest. Do you think there's no end to this market melt up? What happens when corporate buybacks dry up or servicing our debt becomes problematic as rates rise?
Hypothetical: Can I borrow 100k from you? I promise I will put it somewhere safe...the stock market...that should put you at ease for getting paid back, right? I might add that I owe 300k on my credit cards and second mortgage as well! Is everything still O.K. so far? Are you still going to give me the loan? Does lots of DEBT make any difference at all?
Some people believe the government can just print money forever and our markets and economies will be just fine. The problem is the markets themselves will at some point call BULLSHIT and cause the crisis...probably a credit or currency crises. Sure, at first the safe haven will be the U.S. dollar, but eventually...
Example of excess in our economy: I have a piece of cardboard worth apparently 15k. It's a gem-mint 10 Jerry Rice 1986 rookie card that I bought for $4 in 1987 and it's listed on the internet for over 15k. I find that ridiculous, but it gets worse...I know someone who wants to borrow from their credit card to pay for it. I wouldn't doubt they've bought bitcoin this way as well.