And big brokerages have offered this type of account for decades. It's usually called a "cash management account" and works just like a checking account--takes direct deposit, provides checks and an ATM card, etc.
Robinhood's innovation might simply be in calling it a "checking account" so the interest rate looks huge (it's actually small compared to expected investing returns) and marketing it to young people who are suspicious of big financial companies.
> to encourage people to keep all their spare money one-click away from its investment products
These are not mutually exclusive. Robinhood could write this off as CAC that's mitigated by investments into (relatively) safe and low-yield investments. Robinhood could spend a million on Google/FB ads, or they could have an X% chance of losing an amount equal to (3%-bond yield) where X is reasonably low. The (mitigated) loss gives them access to capital and access to customers. If the 3% is permanent, there is no reason for anyone to store their money in a different checking account, as inflation will eat into their savings if those savings aren't invested. Extremely smart move on their part.