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.