Metal actually can be used in a microwave safely. For example, sheet pans with rounded edges and metal bowls are quite safe, as long as you have a decent amount of something in those containers to absorb any energy that the metal reflects. Plenty of prepackaged meals found in grocery stores have a thin sheet of metal in the packaging to change the way the meal heats up in the microwave.

Things like crumpled aluminum foil, a metal fork, or random bits of decorative metal in a dish could cause electrical arcing because charge can build up on the "edges" (e.g. the tines of a fork).

Of course, it's more convenient to use something like glass because we just don't have to give it a second thought (aside from the whole exploding glass problem). And for that, it's reasonable to simply avoid putting any metal in a microwave.