What does reading a great novel or starting a garden specifically accomplish? People do some things for reasons that aren't easily quantifiable. It seems to me that you are starting from the viewpoint that everything has to prove its worth before you accept it, even if millions of people before you have found it fulfilling and worthwhile, which does not seem productive.

If you had never read a book before, and someone was trying to convince you to try it, what could they point to that would fulfill all your criteria? Would it be enough to say it makes you smarter? That's not very specific. It sharpens your thinking? Makes you more empathetic? That would all seem like 'vague undecipherable gibberish' if you had no experience with it. They might resort to saying that it can connect you with a great dialogue that has been occurring for over two thousand years, but as you say, the fact that people have been doing it for thousands of years doesn't make it interesting or valuable.

Seeing a study that some part of the brain responds more quickly for up to 90 minutes after reading or that people with gardens live 0.28 years longer on average would not make me want to do those things more, because those are NOT the benefits of doing those things. You have to figure out what you're supposed to do with your one human life. Science is one tool, culture is another. Neither of them makes the other superfluous.