IMO the biggest reason for this is because an average UFC career is only a couple fights. The guys that stick around and fight in the UFC 10+ times are either perennial contenders or put on exciting fights. So when a loss could potentially comprise 25% of your entire career in the UFC, yeah, it's devastating to lose.
Couple that with a fighter getting older and their chin fading, it's really easy to write them off. I personally would never in a million years have guessed Arlovski would not only make it back to the UFC, but almost fight for a title again.