Possibly because they would then be regulated differently? Currently, there is the claim that all funds deposited are held in accounts by Mt. Gox. So every BTC USD, GBP, etc. can be withdawn and Mt. Gox should not have any problem meeting those withdrawals.
To offer leverage they would need to use a risk algorithm that can limit their exposure to the situation where they have loaned out more bitcoins than they hold, but they cannot guarantee it. I don't want my exchange to be engaged in speculative activity where my deposits are at risk.
Incidentally, here's a related thread:
Leverage comparison SuperThread
- https://bitcointalksearch.org/topic/leverage-comparison-superthread-101081
Thanks, and I agree.
I am the author of the leverage thread, I made it since creating this one, feel free to post in it some more, it needs more information.
I would expect, given that it is presumably profitable, for MTGox to use their profits if they offered leverage to ensure there is always enough money for everyone to withdraw.