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

"Just saying" the above is not what CBT is, though. I've had a lot of CBT and other kinds of therapy, and I've never had a therapist just tell me "so don't do that", or "do more x".

It's about analysing your own thinking(cognitive) and actions/habits(behavioral) in cases where those are keeping you stuck in a miserable situation, and identifying what you can do to change them.

Sure, if you put the most inanely over-abstracted construction on it like "just live healthier and you'll feel better", it sounds meaningless, but the implicit assumption there is that's something you can just decide to do, and succeed in. That's not the case for many people, and those people can benefit from CBT.