[Phillipharryt]

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.

[Phillipharryt]

Replying to myself here because I can't edit it after someone has responded?

I meant wearing our brakes is cheaper to fix that engine parts.

[Scoundreller]

I figured they disengaged because they can’t know if you’re wanting to coast or wanting to brake when you step off the accelerator.

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.

[Phillipharryt]

Automatics don't use a traditional clutch for transmissions, so it wouldn't actually stall, but your other points could be true.