This isn't true for every FPS game. Look at one of the most popular, CoD which has an RPG grind mechanic and levels.
Or even a more hardcore shooter like Tribes: Ascend, which also has a grind mechanic to unlock weapons, classes, weapon upgrades, armor upgrades and even skins now.
The difference is, is that people who play these games, find them fun to begin with, so the grinding doesn't seem like grinding. I feel the same way about SWTOR, I like the grind because it's:
1) not that big of a grind
and
2) I like playing the game