Say you (like many startups) use Mercury Treasury, Rho Treasury, Brex Treasury, etc. Most of these list somewhere exactly what funds they buy into. Why not just open a fidelity account and by them yourself?
The answer is pretty clearly ease of use. Easy to move money from your bank account (likely also with them) to their treasury, easy to set up rules like ("if my bank balance falls below $X then transfer $Y from treasury"), stuff like that
We provide all of these features too! We are not at all asking people to bank with us or spend the time/friction of actively managing their deposits
So if the ease-of-use is the same and the yields are roughly 40% more than the generic money market wrappers out there, we think it's a no-brainer
(EDIT: adding mention that I am OP's co-founder)
But for the majority of founders who want to spend their time building, the fee isn't for ease of use just as this "nice to have", it's for the outcome that ease of use delivers: no need to hire for treasury operations, no manual reconciliation between accounts, no time spent on stuff that isn't your product.
We really think that in most cases, the tradeoff is worth it. But if it's not for you, we totally respect that too.
Also want to note that most treasury products start their fees at 0.6%, which we agree is quite ridiculous hence why ours is less than half that.