So how else do you explain SoftBank's testing results? The testing pool is large enough to not be an outlier, everything else was exactly the same for all their employees - their genetic pool, healthcare system, their cultural habits. The only difference was their level of alertness to personal hygiene. People working in stores were extra cautious of everything while who WFH were laid back and might have ended up slipping up more often, hence the higher cases.
Here's pictures March 24th (and one in late Jan which shows zero masks)
https://photos.app.goo.gl/aUWB5YB6vBnjZk8j6
So no, it's not because Japanese wear masks. I'm not saying that doesn't help but it's clearly not enough given how crowded the trains are and how late it was until everyone started wearing them