How the hell did people sit in a room and decide that it was fine to let the computer have the final say?
That's one good question but I think it's also important to ask why the pilots weren't informed this was the situation and would remain as such until they intentionally turned the system off, and that was done in a way which was counter-intuitive to their training.
When I have cruise control on and I have to slam on the brakes I expect my car to stop. I don't expect the cruise control to keep it going at the same speed. I wouldn't realise I needed to turn it off manually before an emergency stop unless there were a fucking big warning sign on the windscreen. And if there was I'd not drive it on the grounds of "that's fucking ridiculous".
It's hard enough, and counter to muscle memory, remembering which stalks the wipers and indicators are on at times. Having something behave in a way one never expected and were never told about is a disaster waiting to happen.