"Own nothing, but control everything." -John D. Rockefeller
edit: good note from /u/snowwrestler below:
> > It's not the cash/securities that Robinhood holds as part of their business, it's the one they hold for you. You may well choose to hold 100% cash or 100% securities.
> If I'm holding 100% securities, a) SIPC offers me no protection from losses, b) I better be making more than 3% return, and c) I would not call that situation "a checking account."
"I bought 100 shares of Acmecorp, then Robinhood shuttered their doors and said they sold the shares to fund their yacht."
The SIPC's job there is to reimburse you for 100 shares of Acmecorp as of now; they're not going to let investors cherry-pick and say "but they mishandled the account during the one day it was at its peak and I want that price!"
I still suspect that the securities mentioned are your own securities rather than the bank's, which makes it totally sensible that they're not covered by the insurance.