Fish dislike them for the lack of action they create, and good loose aggressive players grow tired of their predictable tight play and see themselves as some kind of class above due to the extra complex decisions they force themselves to play.
However, their reputation is probably undeserved, and some of the most successful players in history have generally been on the nittier side of play - particularly when it comes to bankroll management. Fundamentally, your life is much harder in poker if you are consistently playing mathematically weak hands. Although you can take this too far and only start playing big pairs etc, a well balanced and controlled tight aggressive player is a hard player to deal with pretty much regardless of the stake level in question.
Even some of the biggest winners in the hyper loose aggressive high stakes online games have been on the nittier side of play - it is simply very hard to play against people who usually have a hand, but who throw in enough bluffs that your brain starts to question them at points.
Where nits really break down is when they start refusing to bluff at all. Many players discover poker, realise they get paid often just for betting with only good hands, and start to stop bluffing completely. The problem of course is that poker is a game where any kind of long term predictability is very bad, and as players around them quickly notice their tightness, they start attacking the nits blinds unrelentingly, and fold whenever there is action from them. In all but the poorest standard games ultra tight player type goes broke steadily once they have been identified by the other regulars.
Although watching skilled loose players is often an incredible sight, for most of us mere mortals a tighter approach to poker is usually correct. You face more simple decisions, will have the best of it mathematically speaking much more often, and as long as you bluff occasionally will not have any major leaks. Being a nit is not bad as long as it is balanced well, and this term of abuse often gets unfairly applied to players who are not only profitable, but also greatly feared by nearly all players at any stake level.