[yummyfajitas]

I have no insider knowledge, but here is the obvious answer: fraud is an existential threat to their business.

Paypal's profit margin is a small percentage of the value of a transaction. The potential losses due to fraud can go as high as 100% of the value of the transaction. I.e. their losses can be 50-100x as big as their gains.

Further, the selling point of Paypal is they let anyone who wants to set up shop and use Paypal for money transfers. The tradeoff you make by using Paypal is much lower setup costs (relative to a merchant account) for much higher risk.