Well, (a) just because they invested in Coinbase, which does crypto, doesn’t mean they have as much confidence in crypto as they have in Coinbase—betting on Coinbase is a bet on the volatility of crypto + Coinbase’s ability to win, not a bet on the price of crypto, (b) you have to pay taxes in dollars, (c) their investors probably want dollars.
Why is it a direct conclusion that the investors want dollars? it's currently the only system that is available to Coinbase within the US and they're utilizing that. What is stopping the investors from buying back in with the dollars they gained? (you can't know that for sure either)