Well a) the Apollo team is two people and b) why would they? Reddit’s priority is monetization. Acquiring/hiring the Apollo team doesn’t help that goal.
Monetization and growth go hand-in-hand. I'm an old.reddit.com user (even on MoWeb, I know I'm a psycho), but the way people talk about Apollo is like it's absolutely superior to the current Reddit app. If I were Reddit, and my users loved this third-party product _so much_, I would at least explore promoting Apollo to a first-class interface for browsing Reddit.
For the scale of Reddit as a company, it's likely a trivial deal; whereas the cross-pollination of ideas and UI/UX learnings could easily be worth more than the cost of collaborating.
It is the vastly superior app. That said, Reddit thinks it can get significantly more profit per user itself or it wouldn't be pricing the API so high Apollo had to shut down. I think they are laughably wrong, but they clearly don't see it as worthwhile vs whatever they are planning.