I suspect this is because the US adopted it later when doing live authorizations was more feasible/the norm. I believe NFC was adopted much earlier in Europe and other areas when live authorization was less common, so the limits were implemented. This is just a guess, though.

This is false. Tap absolutely still checks with the bank in the vast majority of cases. I am not sure of the technical reasons that it is faster, but I do know it’s not because of this.