Probably because there is enough competition on the credit card and payment market; do they have a monopoly? Are they driving the competition out of the market?
Another interpretation here: US regulators have been asleep for decades, and they haven’t taken much of a look at a number of companies abusing their monopoly positions.
There's that too, and the US political system ensures that that is kept in place - companies hire lobbyists, lobbyists cozy up to politicians, money is funneled to said politicians' election funds, and politicians are either formerly business leaders, or have a cushy high paying job waiting for them after their term if they play their cards right.
I think with all this (and the cases vs FB, Apple, etc), the tech sector didn't manage to pay and influence the politicians enough. Which is quite telling for the US' political systems.
Enough competition? Collectively, those two control 90% of the credit/debit transaction marketshare as of Jan 2020. Almost every move and change is mirrored by the other -- they don't even try to hide the fact that they practically operate in unison.
Almost all credit card companies have stipulations that merchants are not allowed to charge less for people paying cash -- forcing cash paying customers to pay the same price as those who cost the merchant a CC fee. This is basically a 3% tax we are all forced to pay on everything we buy.
