I suspect Anvil has that credit card requirement to prevent botnet recruiters from abusing Anvil's free trial. (Anvil lets signed-up users run arbitrary Python code on their servers.) By asking for a credit card, Anvil has the information they need to go after anyone who violates their terms of service.

Reflex doesn't have to ask for a credit card because you can try it out locally. With Anvil, there is no local dev, you do all the dev in your web browser, in the Anvil Editor app. It's not just a web framework; it's a Rapid Application Development suite.