The main issue with SEC is that everything that claims or "wink wink nudge nudge" implies that you should buy The Thing from them because it's going to jump in value is essentially an investment proposal that's potentially fraudulent and thus needs to meet SEC regulations.

Stablecoins, where you explicitly state that it's never ever going to appreciate and make you richer by just holding it, don't meet that criteria so SEC doesn't (and isn't allowed to) regulate them.

IRS/tax issues, and AML/KYC (anti money laundering/know your customer) laws still are a potential issue, but that's not handled by SEC.