Most likely, the other providers have not achieved the same level of "success" that Paypal have, and therefore haven't reached steady state in terms of risk losses. Or, they have painted themselves in a corner by advertising that they are much more business-friendly than PayPal, and are taking losses on the chin in the hopes of growing/inventing their way out of the problem.

Why has PayPal not started some sort of program (extra verification, etc) so that legit people attempting to do these things can use them without running into anti-fraud?