But in both modules, the PCs never meet the villain (much less see them) until they walk into the room at the end of the module and roll initiative. The PCs in Burnt Offerings don't get to appreciate how much everything hinges on the villain's background story. They don't get to experience her. They just kill her. Same with Kalarel - the Pcs don't know him, and only know his name mentioned here or there, and they just show up and shoot him in the face with their swords.
The difference between the statblocks of the two villains? The villain of Burnt Offering mentions she can Turn/Rebuke undead, and she can cast Daylight 1/day. These things will not be relevant in the fight, and are there simply because the villain is a cleric and an aasimar, therefore it's there for posterity as opposed to utility.
So not only do we have superfluous things in the statblock, but also we see two different approaches that ultimately end the same way. Burnt Offerings gives great affection to the villain. Details her background, her motivations, her personality. And none of those matter to the PCs, who don't really find them out and end up walking into her room and kicking her ass at the end of the dungeon without any meaningful interaction. Just like Kalarel. The only difference is that the DM gets to read about the former.
The assumption here is that the PCs will encounter the BBEG and roll initiative. It removes any and all possibilities for any other option. Some players may attempt to
talk with the BBEG, to get them monologging, to learn more of their evil plans or to justify to themselves they are in their rights to just kill the BBEG. It is true that probably for 95% of any group, the rest of the stuff like motivation, background etc. does not matter, but for the DMs of those 5%, not having that information means they need to come up with it themselves and usually on the fly when the party does not do as expected and just attack.
I agree that statblocks containing whether they NPCs can turn undead or cast Daylight are pretty much useless, but on the other hand, not giving any information as to some of their background, skills and abilities that might be useful in interaction with the PCs other than combat means if needed it is all up the DM.
The final consideration is, who is the true target audience of any published adventure? The answer is not the players who run through it, but the person who
purchases it, and that is almost always the DM. If the players love the adventure, but the DM finds it difficult to run because they are lacking information they need or want, that DM is less likely to purchase another. If the players love the adventure
and it has stuff the players never see, but makes the DM's time simpler and more interesting, the DM is likely to become a loyal customer.