PF simply expects you to play on your main. Thats it. People set the ilvl to 610 or 615, even 620 now because they expect you to be past crafting gear. So if you have been gearing one job and suddenly want to do what Yoshi P suggested and try healer you have to create your own group but no one is going to respect your group because the ilvl you set is low.

But then there is another problem because raiding is the only way to increase ilvl beyond 620. Realistically after 8 weeks you can start gearing another job. Thats if you are serious and start day 1 but a casual player probably wont clear the 4th fight week one and not everyone is lucky enough to win loot every week even when in a static.

Literally running p7s every week for a 1/8 chance to get a twine or wait 1 month to get a guaranteed 1 twine from 4 books for 1 job just to do it all over again for the next piece when you need 3 twines so at worst after 3 months you can have 1 job geared. Who would want to do that same thing on another job? That is why most people just play one role/job while the other jobs simply get scraps.