Do you have any evidence for that at all? The majority of fintechs I can think of (at least the ones that provide cards, either debit or credit) do so on the Visa or MC payment rails.
One of our greatest failings as a society is our abject failure to teach people finances. It's as critical to our lives as breathing, eating, and crapping, yet the vast majority of people can't even count change let alone understand credit.
Incidentally, anyone telling you BNPL is good for you is trying to milk you. Please just don't and stick to cash/debit or good old credit cards, for your friends' and family's sake if not yourself.
This is not much different from Microsoft fighting open source, Apple fighting right to repair, or Google fighting the CCPA.
As always: Cui Bono? Follow the money.
The downside for the customer is that it promotes overspending, which could be argued as a net negative on the whole. A careful consumer could make use of the free loan, investing the cash elsewhere during the loan period.
[1] https://www.bitsaboutmoney.com/archive/buy-now-pay-later/
The operative quote in support of this from the link, in support of subsidizing those who don’t and almost more damning because even cash users are subsidizing it as the overall price gets raised to roll in the cost of all the people who do use CCs: “Credit card issuers explicitly and directly charge the rest of the economy for the work involved in recruiting the most desirable customers.”
Keep in mind that payments are broader than cards.
I imagine you live in the US.
But if you look at some of the top fintechs in both the US and Europe (e.g. Chime, Revolut, Brex, Robinhood, SoFi, etc.) all help extend Visa and MC's reach.
I'd suggest you read around the threads here to see all the comments about the EU and Japan.