A lot of people here are focusing on ADHD, but I think the author is commenting on a different phenomenon, because they were able to concentrate before and now it's just much harder. I feel it and many people I talk to feel it as well.
It's difficult for me to separate whether this is a result of other changes in my life over time or simply a reflex of ageing in general, though I do think it's a wider societal shift. In the same way that older people find it difficult to watch newer movies (and vice-versa), there is a certain pace associated with culture as well. We have, on average, shifted from books to TikTok in the span of a generation.
I think it is becoming clear that we now have type-1 and type-2 ADHD prevalent in the population. Dopamine deficiency or insensibility are indistinguishable in terms of symptoms.
We are all exposed to so much adversarial noise trying to steer us that people are loosing the ability to focus and to filter.
Dieting and exercising helps. Cut down on exposure to noise from apps, streaming services, devices, etc. And exercise your brain, use it. Read long form text, take notes, write intentional arguments, use these arguments to steer yourself and those around you to create things of value.
I've heard it's habit. You can have a habit of thinking long and hard or you can have a habit of jumping back and forth. You can always reach for the other, but you also have a default for most of the time. I think these days, companies want jumping back and forth because it means it's much easier to grab your attention and get something out of you, without giving you time to fully think through the consequences.