The number of new physicians in the US are restricted because you need to go through residency and the number of residency slots are limited because they have to be funded by Medicare (why ?).

There's a huge lobby (AMA) to keep it that way, to ensure their members salaries remain high.

PAs and NPs don't have the same restriction so hospital systems are pushing to have them handle office visits as much as possible. This also has the advantage of being more profitable because they can charge the same.

In the last 5 years me and my wife have never been able to see a doctor, only a PA, even though we pay the same.