PayPal – use it please

I’m not an impulse buyer in realspace… I plan my purchases and save up for things, all responsibly like, but when I’m on the Internet and I see something I want, I’ll buy it immediately… as long as the site takes payment via PayPal. And there’s little that I dislike more (in my online life anyways) is when I find something I want and they don’t take PayPal. I don’t understand it… most other merchant systems I’ve seen in use are frickin’ ugly and make me want to hide my money in my mattress.

Please, if you want my money (and you do, you know it) use PayPal.

I am curious though why some sites don’t use it. It is not difficult to set up and it lets the purchaser not have to worry about giving out credit card numbers and whatnot. Any legit reasons not to use it?