question about jet ski's

disclaimer: I know most of you guys don't like jet ski's but my gf just has to have one and I can't afford a boat so I think it'd be nice to at least get a pull than no pull at all.

I've been looking at jet ski's for sale online and a LOT of them say "rebuilt" or "new motor" why is this? do the motors on ski's go out a lot? How many hours is a lot for a ski? and which ones are most reliable? I just want to get as much info as possible before I purchase one...thanks