Considering the amount of cancers related to exposures to petroleum products, is this a real question asking why we don’t cook on something that runs on vaporized petroleum products and is lubricated with petroleum products?

Mythbusters has an episode (Food Fables) with Alton Brown where they cook a thanksgiving meal in the engine bay. It went surprisingly well, although figuring out where to put everything and the different cooking temps were tricky.

My wife’s family once baked potatoes in the engine compartment during a road trip, just to see if it would work. Enough fumes leaked out of the engine that everything tasted slightly of oil and exhaust. Car engines are much tighter now, but I’m sure you still get a similar effect.

Op, what’s unsaid here is that there’s planning and enough required knowledge that is a barrier to entry for most people.

Planning: the engine will only get hot enough to do this after a drive of 30-45+ minutes, plus cook time. So for each person, each trip, it means specific planning of what to make, and when you can make it.

Knowledge: engines vary in design, and people vary in their comfort in how much they trust themselves to do things in their engine bay without risking damage. Taking some wire, making a secure spot to hold something wrapped in foil, and it not being in the way of anything AND if it slips or comes lose won’t get caught in a belt is something I feel fine doing, but I do a lot of janky redneck stuff all the time.

Most people barely know how to change their oil, and since the 90s, more and more cars are complex such that they seem designed to be repaired and maintained by someone else not the owner. This makes the simple act of opening the hood intimidating for most people. One survey I found online puts comfort in doing basic cat maintenance at 20% of people. Extrapolate from there an increasingly small group that is comfortable doing weird stuff, ok doing risky weird stuff to their car, and who don’t want to stop at a real restaurant to eat on a road trip (where my spouse will prefer to pee, not the side of the road), and you get a small enough number of people that is not surprising this isn’t common.

https://www.amazon.com/Manifold-Destiny-Guide-Cooking-Engine/dp/1416596232

They have been doing this since at least the 70s, when I saw it. There is nothing preventing anybody from doing it, you just want to be sure the food is wrapped in tin foil or some other heat proof metal container that keeps the bad air out.

It’s too hard to get direct contact with the engine. And thanks to the invention of plastic engine covers all the heat is trapped largely under them. We heated some burritos once on a trim on my moms grand Cherokee.

Also I have a boxer engine so it’s kinda hard to get down to the engine itself, and the plastic air intake doesn’t get very hot.

There’s a diner in town that tastes like this is how they cook their sausage, except it’s done on a 70s era F150.

I’ve used the combine engine to warm up meals during harvest. About 20 minutes on the turbocharger heats a foil pan of stroganoff quite nicely.

Most car trips don’t warrant a stopping for food.

There is very little space in the engine compartment on modern cars.

The engine usually run around 90°C which is not enough to boil water. You could use the exhaust pipe but that is harder to reach.

It’s really inefficient. A stove has all of the heat being concentrated to one spot while a car has a whole cooling system spreading out and dissipating the heat, and cars are more efficient at that than in the 70s. Having enough heat to cook with is generally bad for an engine so by design you would want it to cool before it’s that hot.

I have had a few coworkers who put their lunch containers in their engine bay so that it would heat up for lunch though, that was for a job that had us driving around to 3-7 jobs in a day.

Red Green’s Christmas Turkey Bake

Because RRRRMMMHH RRMMMM noise

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… Nor what parts of it get hot. Or too hot. Or are safe to access, etc. 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. Like, rodent infestation in engine bays is already an issue. Imagine adding (potentially forgotten and abandoned!) food to the mix.

Edit: Another wrinkle I thought of is a lack of consistent temperature control. Your engine is designed to move your car, not remain at a consistent temperature.

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.

So yeah, you can do it to be clever if you like but it’s not cut out to be mainstream activity.