Are you sure that you have four GP friends popcorn? I'm surprised that you haven't taken the trouble to ask them if indeed they are your friends.
As a GP, I find 'bunch of part timers' a really insulting turn of phrase.
As has been explained many times already on this thread, what may appear to the general public to be part time, eg three days, is actually full time because of the total hours worked.
Another mis conception is that some patients think that when the GP is not available to them, that they are not working. Apart from the mountains of paperwork/ admin work, GPs have a range of other roles from teaching to research to lecturing to working part time in the hospital trust, etc
There is also the commitment of CPD which is significant.
Apart from any of the above, GP work is much, much more intense than the huge majority of other jobs. The speed with which we are expected to work, the intensity of the work, the high level of risk and thought processing that it involved is really tiring and stressful.
These are the reasons that many GPs to not work 5 or even 3 days a week. If they were forced to, many would simply leave the profession altogether or emigrate.
WRT lawyer which I think someone mentioned, I have four in my immediate family and other friends/ acquaintances. Those in the high paid roles undoubtedly work very hard (different stress but definitely very hard). However, they get paid many multiples of what GPs earn. One of my relatives mentioned above is on nearer 7 figures as a partner in a city firm and some of her partners are on more. I am not suggesting that GPs in public service should be paid 7 figures but that there is a huge difference in pay.