That's because you only see the levels of classes that are interesting to play. Most regular people are going to specialize in something that keeps them alive and has more use for the general public. Did you want to play a level 7 Dentist or a level 5 Pizza Chef? And feats like Least Painful Tooth Extraction for the Dentist or Perfect Toppings Distribution for the Pizza Chef?

My 300 year old gnome wizard has made it to level 20 six times now, mastering each of the schools of magic before returning to Candlekeep to study the next (and lose all his levels through decades of inactivity)

He's on divination now, and assuming he doesn't die during this campaign, he'll finally master necromancy within the next century