Money market accounts are not money market funds. If you invest in the fund from your regular brokerage account, it is not FDIC insured (but it is SIPC insured like your cash balance).
Not that I don't believe you, but you mind linking some? the best I could find was wealthfront cash, which is isn't a savings account but is giving 4.05% apy. Which is pretty good, but inflation's up at like 6.5%, so it's still a bit of a net loss
Some banks do it as a short term loss leader to acquire new customers. Eg Primus bank recently offered 5% interest, then after a month or two their rate fell back down to treasury APRs
I typically check the top rates for savings, checking, and CDs each week. Deals can appear then disappear quickly. The best rates often have restrictions, but not always.