I’m afraid you’re mixing at least two different things together.

Possible reasons for wanting remote control of home systems while away from home is one thing.

Engineering home systems in a way that they depend (to some extent varying from case to case) on external computers is another thing.

I don’t think there’s any single answer to either of those, but there are a lot of possible reasons.

Personally, I had a use case where I ran a vacuum remotely. I was on a work trip, and thanks to a robot vacuum I returned back to a clean home. That was a convenient and desirable outcome. Others’ reasons may vary.

As for engineering, I imagine reasons must be complex, consisting of at least tradition, cost, state of home network connectivity, and current disparity between consumer and corporate interests.