Because by the time you want to send a wire, you've already decided where to put your money. Plus, it's often a relatively large amount of money an the fee doesn't seem so big.
That explains why everybody puts up with it. It doesn't explain why a startup like Mercury can't come along and shake up the status quo by not nickle and diming their customers like the greedy fucks that are the incumbents.
We give a lot more for free than normal banks. We only charge for wires because right now we have a high fee with our partner. We will fight to reduce that over time.
Missed opportunity IMO.