The average Coinbase retail investor is not going to use a DEX. In fact I would be surprised if over 50% of their users have ever transmitted coins away from Coinbase a single time. Sure the first users were obtaining BTC for productive purposes like buying from darknet markets, but the majority of their users have come during the investment booms. From their S1 they are holding $90 billion in cash and fiat. $44 billion of that is for institutions, but the rest is from retail investors who are buying what Coinbase makes available to them and letting it sit in the app while they wait for it to go up.