I mean, they tried that.  They 
really tried, for years.  First they gave us a Psion core class (they called it a "
Awakened Mystic") in an Unearthed Arcana article.  And after getting feedback, they released a different version ("
Mystic, Take Two") seven months later.  Then they released a third version ("
The Mystic Class") thirteen months after that, before switching--based on our feedback--to psionic 
subclasses instead (first 
in 2019, and then revised 
in 2020).
It's tempting to blame Wizards of the Coast for not getting it right, but they made a solid attempt to give us a Psion class and 
we ruined it.  Subclasses were apparently the only compromise that the majority could agree on.