We certainly could offer cost+ pricing. In fact, that's the status quo in most of the payments industry right now. But as cristinacordova from Stripe points out above:
"We optimize for simplicity though, as we know that many of our users don't want to read an excel spreadsheet detailing the variety of charges we incur from card networks and other parties, which factor into their overall rate. Many of our users have chosen Stripe because we abstract away all the complexity involving pricing."
Balanced chooses to offer blended, simple pricing for similar reasons.
I can certainly steer my customers to use different cards by offering discount coupons for debit. On the internet 1% can mean a big swing in sales.