I'll try to provide an answer, as I'm European but moved to the US. We don't have this "you're in debt" culture that I think you create when you start school in the US. Very early you have to take a loan in order to study, which facilitate the idea of owing banks.
I opened a secured card here in the US, but I never use it, because it just feels weird to "owe" the bank money and not to be able to pay it back as fast as I can. I usually have to wait for the end of the month, and if I mess something up then I have to pay huge fines (that you cutely call APR).
The more you get into this system, the more you seem to collect credit cards. I have friends with dozens of them. What happens if you lose one? Or if you get your wallet stolen? Or if you forget that after the first year you start paying fees on it? etc.
Most of us can live without the rewards if we get other benefits. For example with Monzo, you get added security, you can use it all over the world for free, you can send money to your friends or split the bill easily, you can track your finance, set limits on your budgets, see what was that 30$ you spent a month ago and where it was, round up your purchases to save money, etc. There's just so much benefits that you end up using it all the time as your default card. That's how they get you I guess. But it's so much more practical and easy to use that people have fell in love with their branding as well.