The closest thing to "extra training" they get is fellowship-related rotations, but even this is all things they'll see in practice so they need to know how to handle it initially, if for no other reason than so they know when to offload it to a specialist.
> They are often done by age 24, and ready to help.
I'm sure this is fine for whatever the equivalent is to an urgent care doctor in the UK (bottom of the barrel family med in the US, probably not board certified - e.g. failed the exam or not qualified to take it - or doing transitional residency because they didn't match anywhere), but I'm not really interested in my orthopedic surgeon or neurologist just getting through their training as quickly as possible.
There are lots of ways the US could increase the pool of doctors, and most doctors are probably paid way too much (paradoxically, probably most egregiously at the low end of skill), but "cut out a bunch of training" is a dumb way to do it.