I want my car to be a lot more reliable than my computer. And I want my computer to be a lot more versatile than anything designed for engine control would be.
The fundamental problem is that engine control requires very precise timing of things like ignition and fuel injection, and Windows just isn't capable of that level of precision. Windows is perfectly willing to suspend everything while it swaps data to/from the disk, for example. Other operating systems make strong guarantees about precise timing.
I suspect that there are plenty of car computers out there that don't even use operating systems, at least not in the sense that you'd recognize. The software developers just write code that runs from the very first instruction (no loading separate programs into memory), handles all of the I/O directly, no device drivers, no virtual memory, (maybe no memory management at all) no shell, etc, etc. Keeping them simple helps them stay reliable.