1. Your boss is swamped with work and you'd really leave him/her high and dry for awhile if you quit
2. A new person would require months of training to take over your position
3. Basically you'd be creating a ton of extra work for people to cover your job for awhile
ETA: Also, if you are quitting without having a new job in place first, what is a nice way of saying you are leaving because you just can't stand the work anymore?