While I want to agree with you on principle, I think there are two reasons why they didn't...
1. Monster design is far more art than science and I bet a bunch of the monsters in the book would not adhere to their own guidelines, which was a common complaint regarding the 2014 version. WotC itself admitted they didn't use their own creation rules. Yet the creation rules were often used as the yardstick when people complained about monster design.
2. The rules themselves were based on some odd assumptions and were a pain to use. Creating a new monster was a laborious game of guess and check based on the chart and averaging defensive and offensive CRs with certain variables raising CR and others not. The results were often very bare bones and often required you to finesse the numbers anyway (see 1).
The end result was that building from scratch was not as intuitive as the DMG would make you think. And a strict reading of them could make some very OP creatures "legally" (condition riders on attacks didn't up CR, so you could make a creature that inflicted poisoned, charmed, feared, paralyzed, prone and exhaustion in one hit and it would be the same CR as one who did nothing extra). If the charts were only ever a thumbnail sketch, were they even worth using?
I would have loved WotC to include better rules for monsters, but I think they probably felt the rules weren't useful to new DMs (they require a heavy amount of system mastery to understand) and weren't all that useful to veteran DMs because they often created widely varying results. Maybe they will update them in some future book, but if not the 3pp community has already put out far better systems to do it with.
As an aside, the two biggest uses I had for them was raising and lowering CR for NPCs and existing monsters, both of which the MM including more variety of monsters will help.