Because Jobs are simply better. Ever had a Marauder or Gladiator try to tank in the 40 range? It's terrible, can't even hold hate.

If Yoshida was serious about Classes being for solo, and Jobs for group play, he'd have removed the stats bonus Jobs get and instead have that apply to the party stats bonus; making Jobs weak unless they're in a party. Of course, people would just find that annoying, and it wouldn't really work well with things like the Relic quest at this point, thought it would make Chocobos much more important than they are. So instead we just have Jobs which are better than Classes, with Classes being left in for some half-assed reason.