At least in terms of the population, the EU market is 60% larger than the US, and Europeans are already used to paying for everything with debit cards.
Also, in many EU countries people rarely do chargebacks or expect them to succeed, which makes supporting them much easier.
I'd argue the other side and say the EU isn't a single market when it comes to payment and banking culture. I'd also argue that there are large swathes of Europe that still don't have a card culture.
My personal anecdata on chargebacks corroborates yours, but my own assumption is that the market for a fintech company is much larger in the States than in continental Europe.
To my experience, even the smallest shops in most of Europe accept contactless payments, whereas in the States the smaller ones did not accept card payments for purchases under 2 (or sometimes even 20) dollars. I'd agree that the market for a fintech company in the States is much larger, but this is solely because its current state is so bad.
[0] this is a bigger thing than you'd expect and goes back to my point on culture - unless you want to feel like a tourist standing out, there's plenty of places in Europe still where cash is very definitely implied to be preferred. And don't even try if you're not in a major metro area.