You really think manga represents the majority of the population, let alone niche manga like this?
If it was just this or even a few like this, you would have a point, but as others have already said, stories like this one aren't outliers anymore, they've become so common that they're now practically mainstream Japanese fare.
I mean, you should just never visit anywhere then. Because you can find out some of the craziest things in every country and decide that you do not want to go anymore. Might as well just stay in your bubble where you 'know' what it is like.
Fiction is fiction for a reason. So people can write stories they want. Some authors just copy and paste templates. If you're going to visit as a tourist, most countries are fine. If you decide to LIVE in those countries, different story.
Of course it's all fictitious, but when certain themes keep getting reused by a certain group, there's probably a meaning behind it. For whatever reason, emasculated-doormat-self-insert MCs are extremely popular in Japan, such that they willingly spend their hard earned money to keep the trend going and experience it again in new forms. It's the same with Hollywood and how they keep churning out the same Americaman-to-the-rescue schlock; because it sells (and because it's effective propaganda). The former isn't any cringier than the latter, but at least the latter is easier to understand since humans generally want to feel good, whereas Japan's cuck fantasy trend is the polar opposite, and it only starts making sense when you look deeper at their culture and see the underlying issues that their population contends with, thus resonating with their target audience, which clearly is not the world at large. But since various forms of Japanese literature have been actively marketed overseas, it's inevitable that stuff like this and others like the self-destructively-stupid "kind" MC tropes also get seen by people outside of their target demos, and ends up influencing how we see them.