I'm sure the overall reason is stochastic, with other concurrent contributing factors, but writing off what is generally considered to be the main reason for an acute change in educational metrics (which coincides pretty spot on with am abrupt interruption in home life and school life) doesnt seem reasonable. I haven't heard anything besides the pandemic, being the main driving force for the acute change in educational metrics. Raising a child and reading to them is different than directed education. I value the time I have with my children and read to them daily. This is very different (but complimentary to) than what they would get at some sort of structured pre-k.
This is one of those situations if your looking at the "why," you have to use judgment. All of the data is of course, retrospective, which is not as good as a prospective stuff such as a randomized controlled trial (which it would, of course be unethical to perform). When thinking about stuff like this, I like to point people out to a peer-reviewed systematic review that shows parachute is not associated with survival when jumping out of airplanes (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC300808/). The point is, we can't always clinically study some thing in the best possible way because it's either impractical or unethical, and sometimes you just have to use your best judgement unless/until something more concrete comes to light. We cannot, ethically, do an experiment where we recreate many of the conditions at the start of the pandemic and to conduct an RCT that this drastically affect education (including early education and development).
That being said, I do agree that we should do what we can as parents to raise them, read, help educate, teach good life skills and help instill positive personality traits. I do agree that screen time, distractions, and overall pace of life have been a contributing factor to this for many years, and no-doubt, have played a role at least in the background during this acute decline.
Natto. Literally the worst, IMO. Even a good number of Japanese folks don't like it.