Whilst I'm pretty offended WotC want another $30 out of me merely so my stats aren't outdated on the books I already paid $60 for on DNDBeyond, I have to agree.
The thing is, on D&D Beyond when this book was first announced,
the number one question that everyone had about it was whether it would function as an errata or if you would have to buy its content separate from Volo's and Mordenkainen's original versions. Because, typically on D&D Beyond, if you bought the previous version of the race/monster/subclass and it was reprinted with changes in a future book, it would automatically update to replace the previous version. It happened with the new version of the Goliath in Rime of the Frostmaiden for everyone that had already bought the race from Volo's and Wildemount. It happened with the Theros's updated Triton for everyone that owned Volo's Guide to Monsters. It, most comparatively, happened with the updated, official versions of the Eberron races and magic items when Eberron: Rising from the Last War was released for everyone that had bought Wayfinder's Guide to Eberron. Everyone was asking if the same would happen to the people that owned Volo's and/or Mordenkainen's, and people were
angry. And D&D Beyond didn't give an answer for several
weeks, the staff testifying that they themselves didn't know the answer and wouldn't until WotC told them what to do.
You know all of those people that absolutely hate the direction that D&D 5e has taken since Tasha's Cauldron of Everything? The people that have boycotted every book since that included the updated formats of races/lineages? There's not many of them, but they sure are loud. And they made a big fuss on D&D Beyond's forums, a lot of them saying that if Monsters of the Multiverse's updated monsters and races from Volo's and Mordenkainen's were treated as errata, they would demand a refund for those books on the site.
So, eventually, D&D Beyond's staff got an answer and told the people of the forums that the book would not be treated as errata (even though all evidence from previous examples suggested that it should have been). It's unclear whether or not this was WotC's plan since the beginning, or if they saw/heard of the blowback on the site's community when they were thinking that it would be treated as errata. Or they just wanted to squeeze more money out of the fans that already owned this content and still wanted the updated versions.
I agree that it sucks that it's not treated as errata, IMO it really should have been. But they were getting a lot of backlash from the community and that probably influenced how they approached the book on the site.