People know who to ask to get a quick lgtm.
I don't know what to do about it. I can't make people actually review. I've had unrealistic dreams like holding people responsible for things they approved that had bugs, but anything like that would be supremely unpopular. I could do something like requiring review from the team members that actually review, but they already feel overwhelmed by being the only ones that actually review.
We tried setting soft limits on the size of PRs, but that comes with a lot of PR that are hard to review because the work is poorly divided and doesn't make sense in isolation.