I suspect that there is a certain amount of hiding the man behind the curtain as well. There are reasons to not want players to think about how it works.
The streamlining of monsters to abstract combat space tends to hide the actual size of the monster. The most egregious case is something like a "large" sized snake in 3.5 or 5e D&D. The trouble is that a 12' long snake might actually only weigh 15 pounds. So is the snake large because of its ability to take up space, or medium size because of its reach, or tiny because of its weight?
By not describing the actual physical dimensions of the monster, you avoid the problem of players or GMs of having a prompt to ask these questions about your system. 5e is very much designed to encourage people to be satisfied with a simplified rule set. Thinking about it is discouraged compared to having fun. It's a valid aesthetic choice, and not getting people to think hard about it is a valid design choice, and I wouldn't be surprised if not including real physical measurements was an intentional decision.
