Hmmm - mostly because people generally do not eat enough spinach (BTW we often call silverbeet spinach). https://www.healthdirect.gov.au/foods-high-in-magnesium
we in America often believe if a little is good, more is better the truth is, the dose often makes the poison https://www.chemicalsafetyfacts.org/health-and-safety/the-dose-makes-the-poison/ "In other words, any chemical—even water and oxygen—can be toxic if too much is ingested or absorbed into the body. The toxicity of a specific substance depends on a variety of factors, including how much of the substance a person is exposed to, how they are exposed, and for how long."
Very high dose magnesium is used for “pre-eclampsia” of pregnancy https://www.health.qld.gov.au/__data/assets/pdf_file/0034/139948/g-hdp.pdf
I think in the case of popcorn it just doesn't have any moisture (water) that bacteria needs to gain a foothold. Many products achieve this result with nasty chemicals that we don't want to eat, but probably not popcorn (though certainly it may have nasty flavorings and/or oils in it that are just as bad, and there is reason to suspect that microwaving food can change its chemical composition into something thats unhealthy). Some of the most shelf stable foods are also some of the healthiest, like dry beans, dry rice, anything properly freeze-dried... dehydration, as any Egyptian or Incan mummy will tell you, is the best form of preservation, so long as it stays dehydrated. On an related but unrelated matter, I recently learned that some European royalty and nobility used to eat pilfered Egyptian mummies to gain a then-believed medicinal benefit... thats multi-thousands year old meat that was still, technically, 'edible.'
oil normally goes rancid though "and there is reason to suspect that microwaving food can change its chemical composition into something thats unhealthy" I have often wondered about that one too
They could NOT give me a microwave Chamber to destroy my foods...j m Better yet, THEY could not give me $1M to use a microwave oven.