1. Do these critiques of monsters/monster manuals feel true to others? (Or is this a just me being weird thing)?
Well, if you're someone who makes bespoke monsters every time -- and I do a
lot of this, myself -- big premade books of monsters can often be of less value, for sure. I think Monster Overhaul isn't primarily aimed at those people.
That said, as I've been pulling out as I work my way through it, there's a
ton of interesting ideas in the book that are useful even for people who won't ever pull from the book directly. The dragon entry, for instance, cries out for coming up with more dragon types to put on its chart, now that Skerples has helped show how weird dragons can be.
For myself, my preferred OSR engine is Shadowdark and while it has a decent-sized bestiary, it doesn't have everything and many of its choices are TSR D&D flavored, which at this point, I find pretty vanilla and underwhelming. I like my OSR to be darker and weirder, and nearly every entry in the Monster Overhaul has at least a few nods in that direction. (The Monster Overhaul druid, for instance, I find to be completely compelling -- a wild man of the woods who is clearly dangerous to be around, even for "allies.")
2. Why do people who love Monster Manuals love them?
This isn't necessarily me. I have some very specific monster books (most of Cawood Publishing's Monsters of the ______ line, and a book of Latin American monsters), to flesh out specific campaigns I'm running. I'm also saving up my pennies to get Monster Manual Expanded to flesh out the 5E Monster Manual.
But I think most of the people who are bestiary collectors like seeing the wide variety of crazy ideas, partly for the inherent fun of it, but also to surprise players who tend to memorize the core monster book of the game they're using.