That is debatable and debated, lacking these details did not prevent 5e from being the most successful TTRPG ever, so I don't think that there is a "need". Some DMs might appreciate a bit more help, understood, but as this current discussion shows, there are probably more opinions than there are DMs, and it is so dependent on the circumstances that it is, like everything in 5e, up to the DM to customise it. You can't be a DM in 5e like you were in 4e where everything was pre-chewed for you, it's the nature of a much more open game, you need to learn the ropes and how to fill in the blanks.
If DMs need help or explanation of how to run the game there are a ton of resources. Blogs, podcasts, how-to videos. Some of this could have been included in the core books, but there's no way to have enough page count to even begin to match a fraction of the advice on rule implementation and guidance that's available with a simple google search. In addition, any advice would have been taken as official "this is how you're supposed to run the game" and we'd be arguing about that.
It's impossible to come up with blanket rules for every group in a game as flexible as 5E. It's why we have "rulings over rules".