Because I can create a cryptocoin with 2B tokens, sell one coin to friend for $100 and have a $200B market cap. It's got nothing to do with reality. Anyone (literally) can create a $200B market cap coin by forking a github repo.
https://signalvnoise.com/posts/2585-facebook-is-not-worth-33...
It's worth almost 17 times more than that now. Market cap is not a perfect metric because it doesn't capture the depth of the market willing to buy but coins don't make that any different.
It's an absurd metric made worse by the fact that more coins are issued every 5 mins. If price remains the same virtually all coins will go up in marketcap regardless.
What company issues new shares every single day?