what is actually involved in setting up a payment processor
you need $80 million to get licensed.
you need partnerships with underwriting banks.
you need agreements with payment processors.
you need cardholders to somehow stay solvent with aforementioned.
you're talking $100+ million to even get started and then you better somehow have enough volume that scraping 3% off each tx keeps you afloat. I imagine you'd probably need to charge even more than 3% to get off the ground but having higher fees means fewer businesses will even take you.
Then you're competing in an ecosystem where there is no actual advantage. One of the main reasons why you can't pass off fees to the customer but can do cash advantages is that when you do 5% off for cash, you are treating all card networks the same. If you transparently showed your MasterCard fee is $3.20, your Visa fee is $3.50, and your Amex fee is $5.50, then consumers would choose the lowest priced network if they didn't have cash.
But that's not how it works, by design. If they're checking out, do they choose the MasterCard, Visa, Discover, Amex, or KiwiCard? Well, there's no distinction in practice, so why not just use your Visa debit card? That's what most people would do. They might use their Amex if they're saving points for a trip or something. That's the consumer's mindset, not what is the most fair to the business, or what creates the most liberty.
Here's a good example: Discover card. Discover is a merger of Discover (Sears, 1980s) Diner's Club (International, 1950s) and JCB (Japan, 1960s - which is why Discover card is accepted throughout Japan and every 7-11's ATM also lets you take cash advances from Discover). Discover has the lowest market share of the main four card networks at under 5% total marketshare (which didn't stop them from banning Dick Masterson's New Project 2 from accepting cards from any network, by the way).
So despite competing for decades and being a merger of several smaller card networks, Discover has never seen significant market share. In fact, listening to the radio recently, I have heard actual advertisements from Discover that go along the lines of:
"ay gurl whatchu doin?"
"shawpin gurl"
"wit cho discover card??? nobody accepts that!"
"it ain't tha nineties anymore gurl! Discover is accepted at 99% of retailers now!"
"sheeeeeeeeeet"
That's not hyperbole, Discover has been around for half a fucking century and is still trying to convince people you can use it to buy things despite being owned by fucking Capital One. How do you think KiwiCard fairs?