That might partially because it takes a very very long time before a species is declared extinct. The actual document[¹] is interesting because it includes some responses to peer reviewers and goes more into methodology. The species that were just declared extinct haven't been seen for decades. In some cases, the animal has probably actually been extinct for 50+ years, they're just (understandably) very cautious about declaring it. It would be awkward to have another coelacanth situation.
That's a good point, although in this case you can count the number of wild vaquitas on two hands. I imagine it's much harder for birds, insects, and fish, but for dolphins, they stay by the coast and need air. I can't imagine a species coming back from such low numbers.
1: https://www.federalregister.gov/documents/2023/10/17/2023-22...