The stability factor is very much an issue as others have pointed out. You could be fired or relocated with very little notice at the whim of your employer's major partners.

Also, you are much more susceptible to economic pressures from entry-level pilots. You could be 50 yo, with a wife, 3 kids, and a mortgage...you need the $90K, but when that 23 year-old PFT with 200 hours is willing to do YOUR job for $16K your company is going to want some serious concessions from you.