Couple ideas:
- Because you still might need a way to filter out bots. Crypto puzzles don't filter out the bots, it just delays them a bit.
- Because there are not many good, maintained solutions.
- Because most devs do not care about being independent from external services such as Google and Hcaptcha 🙈.
P.S. this comes from someone who created an own self-hosted captcha solution ðŸ¤