We are using Paypal because Apple has offered it - and if they offer it it has to work. Simple.
Don't forget that CCs aren't in every country a common option to pay with, limits here are often lower than "you" have and you have to give them good arguments to get a higher limit.
Also some may have a balance at paypal... why not use it?
I didn't realize that getting a CC company to commit to a few thousand Euros was such a big deal. I can buy at least two fully loaded nMPs (each with a couple of high end 4K displays) on credit, and I'm not rich by US standards.