Yeah, it's true. I think that's why I really enjoyed those old Creature Crucible products of the 80s/90s, or the more recent Fizban's Treasury of Dragons and Glory of the Giants. The reception for these products has been rather lukewarm, but I really appreciated how--through a narrowed focus--those books were able to provide both a rich lore and abundant stats. I'd love to see more, like:
Nihiloor's Codex of the Illithids: a deep-dive into mind flayers. Their history, society, mythos, and more, complete with stats for all the many (MANY) types of illithids from the Elder Brain all the way down to the smallest tadpoles, new rules options for psionics and aberrant character backgrounds for the players to enjoy...
Slarkrethel's Guide to the Undersea: Everyone's favorite kraken gives us a tour of the underwater kingdoms of the realm, with lots of info on geography of the Purple Rocks, the struggles between merfolk, tritons, sahuagin, and kuo'toa, the eldritch origins of the aboleths and krakens, loads of history and lore of undersea creatures, stats for hordes of aquatic creatures, new rules options, some new druid subclasses...
Nerull's Necronomicon: A deep delve into the world of the living dead. Obviously we would need new spells and character creation options, sure, but also: cultists and necromantic factions, the lore behind the ancient mummy lords, NPC liches and vampires ready to pounce on your unsuspecting heroes. Throw in some maps of tombs, vaults, and temples, maybe a couple of pre-written adventures...
Contact me directly, Wizards of the Coast. I have oodles of ideas, and they can be yours for a small consulting fee.