Because that part is wrong, apart from the underlying inner working being dissected in labs (the cognitive science part), the therapy part it is well studied in the wild - and it works.

> and the results of therapy I come across always seem extremely underwhelming.

So you mean anecdotes? Of course you recognize the underwhelming results - the people where it worked well are less noticable, by definition.

> Maybe for some cases like severe phobias there is something to be solved, but do we really need to be paying thousands of dollars a year

That you have to pay money has been a decision of the society you reside in. It doesn't have to be this way, and in fact it isn't in most industrialized countries.

> for a therapist to say "hey, maybe if you'd invest more in hobbies, social relationships and a healthy lifestyle you would feel better"?

That is a strawman, not CBT.