I think learning to oil paint could be a very meaningful experience. Relaxing to watching Bob Ross videos and paint along with him.

I do also feel there is this sort of "cult" of self-improvement going around. Like if you're not spending every waking minute of your life learning some new skill or marketing yourself or trying to get a promotion, then you're wasting your time. It's very toxic.

Doing things that you find relaxing should be accepted, even if they don't teach you anything or improve you in any way.

No, but building sets does have other benefits.

There's some zen to the act, like model or puzzle building. But you can also observe and learn techniques to add to your own builds.

Knowing all the ways Bionicles are put together can help you turn a Porsche into a full transforming Autobot Jazz.

It isn't even a puzzle if you've been given the instructions on how to build it.

I think you're making the mistake of assuming that your values are universal. I don't think there's anything intrinsically meaningful about painting, or doing any other creative activity. If someone enjoys those things, that's awesome - but not everyone does, and there's nothing wrong with that.