I don't have to invest any time at all. I just use the (US) card that gives me the greatest benefits, be it cash back or services. Usually I just look at the reward rate, which is a base 2% for me right now going up to 5% for some things.
I love it. As someone who never carries a balance I get paid by banks for doing pretty much nothing at all.
And I don't worry about US retailers, I don't live there.
I should add that rewards are not the best benefits. Sign up bonuses are much more lucrative, running to hundreds of dollars per card, and can often be repeated. Same applies to bank accounts.
You're not paid by the banks you're paid by other, usually poorer, customers
Imagine a world where the pipeline from extra fees back to hoop-hopping cashback didn't exist and your services were just cheaper by the same percentage points instead. It's designed to make money off people slipping up instead of serving customers.