Not sure why? It’s pretty straightforward for US to use the E-gates in the U.K. as a reciprocal arrangement. (Which we don’t get over there).

And to be fair, the last three times I’ve been to the US, there have been significant queues and it always requires a check, f/prints or photo and a stamp which takes a finite amount of time. The staff are usually sullen at best and downright rude at worst.

Personally I’d suggest that it’s the US that needs to up their game in this respect.