You are missing the fundamental aspect: Money
WOTC knows people want more classes.
Bt those classes wont be in the Player's Handbook
The PHB is the highest selling RPG book.
Printing a book with big new classes lowers PHB sales.
So WOTC created new classes.
And again 3PP don't build the same classes. One makes the Warlord. Another makes the Shifter. A third makes the Apothecary. A fourth makes the Psion. A fifth makes the Anti-Paladin. So you need to buy 2-5 more books to include them and
- None of them are supported by DDB
- None of them are supported by WOTC books
- None of them are supported by other 3pp books
- None of them are supported by setting books
- None of them are supported by setting neutral splat books
How many Blood Hunter feats and subclasses are in Tasha's and Xanatar's or your general 3pp kickers?