Over-simplification: As a "not-bank" they can't engage in fractional reserve banking. This leveraging of money is what allows banks to truly generate profits. Acquire deposit assets at 1%, lend that same money out at 3-5%, profit on the spread. If a bank is offering a low rate that means that either their lending arm is not able to effectively use deposited capital or their overhead as percentage of that spread is too large. Higher rates signal that the bank is attempting to gain capital in order to supply their lending side.

Simple's funding likely only increased their runway while they were building a product that I'm guessing was net-negative. I don't feel like they could generate enough revenue to cover their burn rate using their holding bank strategy. If they could I suspect they would have continued down this path as a fearsome (future) competitor.

Once some bank builds the underlying technology in-house correctly they're going to be nigh untouchable.