This is a return to 3e based design, where the goal is to codify everything.........or as much as possible. A rule for everything, and for everything a rule. Which is simply not how 5e plays, it is intentionally much more rules lite than that.But IMO there's no reason for the rules not to try to have an answer for as many situations as possible, so the DM is not constantly required to make rulings to fill in gaps the designers should have addressed.
That said, there is a compromise. After 10 years of play, there are certain "interpretations" that dms have to make over and over again on the same scenarios, and those could certainly use a little shoring up. Illusions are a good example. I don't expect a tome on illusion handling, but there are a few pretty common themes with illusion that could warrant some rules....or at least a few really good examples. the classic "illusion wall that I can fire through but my opponent cannot" is one easy example.