There are very few things in my life that are as productive as the gym. It helps my memory, my mental well being, my cognition and my health. I'm unsure of why you don't consider exercise to be useful. Can you explain? Below I have included a quote and link to a study that looks into exercises effects on cognition. As for people's zealousness for exercise on this site, I think its a result of people wanting to help others. For many, getting into exercise was the single best decision they felt they've made in their life. And since almost anyone can make that change, and do it for free, they're trying to spread the news. If I spend the time to recommend something as small and inconsequential as an application to someone, it seems absurd that I wouldn't recommend something like exercise. But that's just my two cents

"The evidence accumulated so far indicates that exercise is a strong promoter of cognitive health in humans. "

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3951958/

When we realized physical health contributes to mental health and mental health contributes to discipline which contributes to physical health.

Increasing the physical health of all people would be the most productive things we could do as a society.

Nothing else would remotely come close.