Again, from my research, it is because the partner banks will not allow them to.
And, now that Stripe is quite big, don't they have some leverage over the banks to be able to say they do want porn sites et al. to be able to work with Stripe?
As for Stripe's size, they are a tiny, tiny drop in the bucket compared to the world markets. Heck compared to a U.S based hedge fund they'd be tiny. I'm guessing they have very little leverage.
We rarely get chargebacks. Maybe that's because we focus on a different kind of customer.