This is a great question, I actually don't even think it's strictly a matter of "more" yield.
To dig into your example a bit deeper, there are a few general differences with an (eg.) Unit vs. Bridge.
- With a BaaS like unit, you're often actually forming a partnership _not just_ with Unit (a software provider), but the partner banks Unit works with. More specifically, you're operating two sorts of programs with the partner bank: programs around "money transmission" (_moving_ money on behalf of customers) and around "stored value" (_storing_ money on behalf of customers). Each of these programs tends to be pretty involved - as is having to be in a three-party agreement etc, working with an old-school bank, under legacy banking constructs, etc.
- With Bridge: Bridge is your single partner. Bridge _itself_ has partner banks for the sake of both banking + money movement, but when you store customer funds as stablecoins in a crypto "wallet" Bridge spins up, it is operationally different than if you were to store them as fiat in a customer-specific bank account you opened at Bridge's partner bank, under a classic FBO/DDA program. The partner bank Bridge using is more involved in the money _transmission_ piece, when you want to receive customer funds in as fiat, or push customer funds out as fiat - but the funds being stablecoin at rest would seem to reduce the burden here.
So yes that might result in some cost-saving, but it's also just (vaguely) _easier_ to do from a technical/operational POV.
(Some of the details above may be wrong / vary by provider / etc - but having worked with both of these sorts of providers at some length, this is my "felt" difference and high-level understanding).