McDonalds doesn’t sell veggie burgers because people don’t buy them. That might have something to do with the fact that most pre-made veggie burgers taste vaguely like cardboard. But now that startups like Beyond Meat and Impossible Foods are coming out with plant-based burgers that taste close to the real thing, it’s possible — probable, even — that fast food chains will soon offer virtually indistinguishable vegetarian versions of their beef burgers. Beyond Meat has offered meat alternatives like veggie chicken…