And why is that?
Because I don't want to create an account and send money to it. If I buy a game I either want to pay it in cash or want them to withdraw the money from my account.
I already have a click&buy account which does what I want just fine and I've never felt comfortable with paypal. Also I'd never buy a game online with anything other than steam as I don't want to have my key lying on my email or spread out over 5 different accounts for 5 different stores.