You can level to 99 in a couple days with or without a cleave party, so why pay for something you can get for free?
Opportunity cost? It isn't that hard to figure out why someone would want to pay for something they don't have to do on their own. You can get cleaved while you are at work/sleeping, come back and then do other stuff instead of leveling during your actual playtime.

If your LS is, for example, selling NNI drops for 5M a pop or whatever, it isn't that painful to separate with maybe 1.5M or whatever to cleave your character up from 30. Hell, even if I only just made money through Dynamis every day (~1.5M every time solo), I really wouldn't mind trading one of my runs once in a while to get another job to 99.

For the record, I've never paid to cleave up any of my jobs since I do prefer to xp them myself anyway, but it isn't that hard to see why people would prefer doing that instead.