They were never intended to have 100% of the teller/check stands open.
It’s for surge and holidays, if you go in on Black Friday or other super busy times, you’ll see a vast majority opened.
It also makes counting easier, if 1 person uses a drawer and it’s off, it’s easier to hold a person accountable, rather than if 5 people used it
Most of those online only banks subcontract out the banking part to a real bank