[jacobolus]

These activities are "non-useful" in the sense that they generally don't bring direct financial rewards to the individual people who spend time on them (beyond modest salaries for teaching/research for those who work full time on it). However, from a societal perspective, such activities in aggregate often compound and have very high leverage and therefore disproportionately large benefit, especially compared to purely consumptive hobbies. The rewards are not carefully accounted and are reaped by miscellaneous strangers, sometimes far in the future.

[BeetleB]

As the OC mentioned in a sibling comment, it's actually the societal aspect that leads people to abandon pursuing things like mathematics.

When I was in grad school, I was surrounded by people who had these interests. I could work on a cool math problem, and people would be interested. I could have a conversation with them about it.

Fast forward to the "work" world, and there's not a single person I know in my city with whom I can have these discussions. And trust me, I looked.

As the OC said, it becomes an incredibly lonely life. If I want to study a typical grad level math course after work hours, it can easily consume all my free time (trying to solve a given problem could take hours).[1] After a few years of trying, I had to abandon it for the sake of my mental health.

In retrospect, there probably is a middle ground, and if I return to it, I'll aim for that middle ground. Intentionally choose topics I can do in bite sized amounts, and topics that I can share online that have a higher chance of engagement.

[1] And this was when I was single without kids. Imagine how much less time you have with kids.

[lupire]

On the Internet, the city has billions of people.

Rural towns are devoid of most things. Every urban area has math PhDs.

[BeetleB]

> On the Internet, the city has billions of people.

Quality of conversation is very different. Here I am, talking to you randomly. But I can't keep pinging you to discuss math.

> Rural towns are devoid of most things. Every urban area has math PhDs.

Most of whom have "moved" on and do not want to do math in their free time.

[nadam]

It is not just about the financials. It can be a very lonely activity to create something that other people do not care about, or just very very few people care about. (As it is described in the article.) Besides financial reward, social reward is important too.