Try out Singapore and fly for example to Phuket. It’s a very lean process with optimised waiting times (read: nearly none) and walking distances on both sides.
It can be done, most airports are just amazingly inefficient. No idea why that is.
At least in the US the problem is that you can’t afford to shut down existing airports and so a lot of the remodeling ends up being hodgepodge and convoluted extensions and fitting whatever goes into a nook or cranny.
The last major airport to be built in the US was DEN in 1995.
To sell meals, drinks and other goodies. Tired travellers that are forced to wait are easy targets. Add in the occasional perfume store and Bob's your uncle.
Because the UK really doesn't do regional airports. If they did it would help, but basically I dont have many option outside of the London three or Manchester.
Funnelling everyone through such a small number sounds efficient for the airport, but isn't from a travellers perspective. Heathrow and Gatwick, are massive.
I haven’t been to UK airports, but often I find one of the more prominent bottleneck to be central checks per terminal instead of distributed checks at each gate like they happen at more efficient airports. The latter is a major improvement: You can directly go to the gate and bag checks happen there.
It can be done, most airports are just amazingly inefficient. No idea why that is.