I think it's important to not conflate being "sexually attractive" or "hot" or whatever with being "pleasant looking." Someone can be overweight or thick or whatever and still be pleasant to look at from a purely aesthetic point of view. I don't think anyone believes that the audience should want to screw every member of the cast.
That said, in terms of "romantic" roles, I think a lot of people have some weird biological thing going on where even though in the back of their minds they know it's pretend, their animal brain is more likely to give approval to "attractive" people "mating" (even if it's just singing a duet and kissing a little bit). So the average audience member is still going to want, say, Elphaba and Fiyero to be played by "hot" actors, because biologically, their brain is more accepting of that. "Good job! Go make beautiful, healthy babies! Advance the human race!" Now what a certain group of people or society are inclined to think of as "hot" changes throughout the ages somewhat, and is always ultimately up to personal preference, but I do think that health is a big factor. If an actor is visiblly overweight to the point that it makes them look unhealthy, I think that can be limiting, whether they are auditioning for romantic roles or not. The average person, if they had to choose, would rather spend two hours watching a play or movie with people who are not overweight, as opposed to people who are. Especially if those people are overweight to the point of obesity or being visibly unhealthy.
I'm not saying any of these things are "right" or "okay" or whatever. Just as a sort of amateur psychologist (and an actor always trying to figure out what roles are available that I might be "right" for as well), this is what I make of why certain roles are cast the way that they are. Human beings, like most other animals, judge each other on how we look, both in real life and in theater/film. It's biology.