Because credit cards are a scammy middleman that has dug their nails deep into the political and financial worlds that lets them bully merchants into using them while pretending to be a benefit to consumers. In reality they just drive the price up for everyone, and then push a few percentage of the markup they pocketed onto consumers to appear useful. Consumers lose in paying higher prices, merchants lose by money being siphoned away to a 3rd party.
Credit (and debit!) cards solve the trust problem of online shopping and as such are vital to competition: I know I can get my money back if I don't receive good or services anywhere; if that weren't the case, I'd probably only shop at a very small number of highly trusted merchants.
Credit cards, in the US implementation where the issuer collects ~2% from the merchant and pays their cardholder a kickback from that, are both a cost to cash payers and very inefficient in general, but that's not the only way of doing things. The EU capped interchange fees at 0.3%/0.2%, for example, and purchase protection is still available just like before.
This is slowly changing, but will take time as merchants switch to accepting instant payments ("pay by bank"). My understanding from the merchants my org interfaces with is they are surcharging credit cards to drive spend to cheaper rails (when available).
(if ACI, FiServ, or Jack Henry processes your payments today, you can accept instant payments without much more effort, inquire with them if interested)
