Why? Because it's mean?
Well yes, it's mean to let your Sim be a loser. It's one of those traits that aren't very nice. So if I play with the Hart Family or Gobias Koffi, that trait is replaced with something nicer, and more suited with their other four traits and overall personality, with the simple Shift+Click=Modify Traits for Active Sim.
Gobias gets Commitment Issues (because it works well with the Flirty and Childish traits), and ever since I installed Generations, Dorie gets Rebellious, because of her bio stating she's not always on the same page as everyone else.