A Good Adventure is Hard to Find
A wizard who lives a long natural lifespan will have perhaps 4000 weeks to adventure, research, scheme, effect schemes, and conjure the object of one’s vices. Most wizards would also agree that they would like to attain as much power as possible to further their pursuits. Not only is a higher level better because it offers more intensity and variety per moment, but it also allows the wizard to gather the resources to pursue his goals. For example, a wizard researching spends perhaps 500gp per week, making magic items costs 3,500gp per week. A wizard needs to pay his cost of living, too. Servants, fine liquor, and silk sheets would be nice to have, too. How to raise the funds when even owning a magic shop earns only 19gp per week and working as a 10th level wizard pays about 14gp per week? Skilled work pays a little better. Those downtime rules look broken to me, but I will have to dive into that rabbit hole at a later date. Let it suffice to say that as written, a wizard would have to own 185 magic shops to support himself at making magic items full time.
Unquestionably, the best answer to this funding problem would be adventuring. The profits are enormous compared with ordinary work or capital. With only modest breaks of down time to reequip, adventuring in a world where magical potential comes from overcoming dangers to accomplish goals and accumulate money is both lucrative and empowering. Many campaigns I have played in have effectively leveled my characters 10 levels or more in a year of in-game time. I have always preferred a bit more down time per level when I GM, but fast leveling (as measured by in-game time) is certainly a thing in official Pathfinder campaigns. Yet Golarion and the settings I have played in are not full of high-level 22 year-olds, nor are old NPCs always high-level NPCS. If being a 10th level 22-year old was as simple as slaying enough people with money, I strongly suspect a shortage of brutal 20-year olds would not be the limiting factor on a world full of high-level brigands. I suspect that what limits leveling is that appropriate level challenges become rarer and rarer as level increases. A 4th-level brigand and his gang hiding beside the road find fewer level appropriate travellers to rob, etc. At a certain point, adventure slows down and you have to spend your time less lucratively. What is a wizard to do when he has spent all his adventuring money, there are no adventures to be had, and he is worried about making rent on his villa and paying for experiments?