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.