There are quite a few open source or royalty free Japanese fonts (Google Fonts has 50[1]).
But, as everyone else has mentioned, font usage in games (and most creative visual works) is more particular than just the bare minimum of "does it actually render the glyphs". Imagine if all text in your favourite game was all Times New Roman, it would make the game worse.
Alternately, imagine if all text in your favorite game was IP owned by some random third party that could ruin everything one or two years down the line.
Perhaps it is time for more people to invest in royalty free IP? We are seeing a bit of a tragedy of the commons type of situation going down, right now.
I grew up in russia and every localized game always had godawful ugly thin fonts. I would always watch foreign gameplay videos with jealousy cause they had beautiful latin fonts.
Now it's been years since I played a game in russian (almost a decade at this point I think) and I am so glad I don't have to put up with that anymore. Once in a while I see a screenshot from a cyrillic-using language translated game and probably half of the time the fonts are still bad.