Why does a bed need to run Linux? Why? Of all the possible timelines, we live in the dumbest. What was wrong with a plain old bed without 1GB of RAM and a full OS running on it?! It is the same everywhere. Finding a washing machine that was not WiFi-connected was a chore and I dread doing it again in ten years. As a person who's broken into O(1000) "smart" devices (for fun and for profit both), I do not want them in…
Sounds like the problem isn't Linux, it's insecure development practices. As mentioned in sibling comment, Linux development is far easier to hire for, iterate on, develop updating mechanisms for, etc - specialized embedded development is less popular.
No, it's making devices "smart". There doesn't need to be a wifi-connected computer inside a washing machine, cooker, or fridge. In fact all these things can run without a computer in them at all, and they're arguably better for it.