Automatics disengage the driveshaft to save wear on engine parts. It's an active decision, not being "too dumb". Wearing out brakes is cheaper to fix than wearing out your brakes, and people like longer lasting, cheaper to fix, cars.
I meant wearing our brakes is cheaper to fix that engine parts.
If they always assumed you wanted to brake, that would really hurt mileage.
And it would have to disengage anyway at some point to avoid stalling.
Engine braking is more RPMs in aggregate, but nothing particularly bad for an engine.