Difference is the FDIC insurance, mainly. Being member FDIC means that you have to go through a ton of regulatory requirements to be sure that you're financially stable. A bank can offer whatever interest it wants on its account if it manages to stay in compliance. If Robinhood were to become member FDIC and offer 3% I imagine there'd be no qualms about it.

SIPC on the other hand is more relaxed because it covers less. It only covers when a member broker becomes insolvent and is unable to return a security to you that you own.

There are similar funds that invest in short term treasuries and often that you can write checks against, however they don't offer 3% and they're very transparent that you own these securities and are subject to the (very low) risk that comes with owning them.

Robinhood's marketing sounds a lot more like a standard FDIC insured bank account but they're saying it's SIPC insured instead, and the SIPC is confused about what securities they would actually be insuring