One reason certainly is that Coinbase charges very high fees, namely around 0.6% of the traded amount. The other reason is that crypto is traded a lot. If 1/6 of the entire crypto market cap trades on Coinbase every quarter (and that seems to be the case), then Coinbase has earnings of 0.1% of the entire crypto market cap every quarter, or 0.4% per annum. Apply a 25 P/E ratio, and you get a value for Coinbase of 10% of the crypto market cap.
Maybe because traditional investments have a lot of uses. Since crypto is pretty much just speculative right now, it makes sense that the exchanges are the ones capturing that value.
However for crypto exchanges ... well ... what is going on here?