Okay, but you are comparing apples and oranges. One is made to certain tolerances, confirms to certain standards, has QA applied to it, comes with an implied warranty, and is produced in lower quantities.
Additionally the roasting pan also isn't exposed to the elements, with vitreous enamel susceptible to "crack or shatter when the substrate is stressed or bent", so it wouldn't last all that long when it gets a whack from something hitting it underneath.
The real question you should ask is why is the roasting pan so cheap? Which then leads to paying starvation wages, lack of regulation with regards to pollution controls of the factory that made it, etc.
But by all means, bolt a roasting tin to the bottom of your engine and report back the results. You might get roasted a few more times though 😉