Lots of People Are Losing Distance Vision, and No One Knows Why

Actually, we've a pretty good idea why, from looking at countries with very, very high levels of short sightedness. The article itself actually references these (mainly conducted in South Korea and the US).

It appears that it's down to people not getting enough sunlight as children. The sun is, obviously, very bright, which forces your iris to have to contract and relax a lot, which keeps both those muscles and the lens in good health.

With populations increasing having children that stay inside, they're not exposed enough to that much harsh, bright light, so their irises weaken, leading to short sightedness.

Caveat - a lot of the research of this is looking at factors that correlate with the levels of vision reported, so the cause is being inferred, rather than know. However, it's a decent hypothesis, and the more recent meta-analysis studies done seem to be supporting this.

The problem is that once it's happened, it doesn't appear to be something that can be "cured" by re-introducing bright light exposure.