In the 1970’s and 1980’s there were several books the either had characters that did it or promoted it.
Why is there no cooking tray in my new car’s engine bay?
Is it dangerous? (It would be less physically dangerous if there was a specific spot for it.)
I imagine the majority of the current crop of motor vehicle owners are not quite smart enough to realize that the hood even opens to allow access to the engine bay. And in our modern litigious society there is simply far too much that can go wrong with this to make it worthwhile for any manufacturer to include as a deliberate feature.
The utility is also rather limited when you have access to a microwave or a convenience store. Or even a convenience store with a microwave in it, as many do.
While I don’t disagree, this is a line of thought that is used to talk people out of everything, including many things we do commonly.
Temperature control is less important for many baking items (potatoes, some casseroles) than modern recipes indicate. Modern temperature control is “easy”, but not needed for many things. Temperature control is critical for a few things, and those would be a bad match.