@kanaloa This Grail War specifically was originally a parody manga (Fate/Koha-Ace) that made jokes off of a lot of the tropes in other Fate series. The use of extra Japanese characters in that series was intentional since the setting was supposed to be based on a specific twist where Japanese servants were capable of being summoned. Plus it happens in Japan, and doesn't really focus on the greater aspects of WWII, it was just for setting purposes. The servants present have almost never been part of the current historical events outside of the region where origin fame grants Servants from that place a boost.
And if we're talking about the more general Fate series, yes there is bias since it's made by a Japanese company and Japanese writers for a primarily Japanese audience. Don't know what you expected.
But there's still a ton of servants from basically every inhabited continent except Australia and nearly every culture that has characters famous enough to have permeated general media.
To specifically address China, we currently have seen depictions in Fate of 17 different servants from Chinese history and mythology, so it sounds more like you just are ignorant rather than bringing up legit concerns.
For Egypt, the reason we don't have Anubis or Ra directly is because Divine Spirits can't fit into Servant containers unless they're very weak or possessing a human as a vessel. We do still have 3 of the Pharoahs, though, and Ozymandias is considered a manifestation of Ra.
Cambodia doesn't have representation, but it's far less famous in terms of general media than most of the other mythologies and histories that have been drawn from, and there's nothing preventing Cambodian heroes from appearing, should the setting and catalysts make sense and a fitting character for the story be found.
I guess you could be trolling since you aren't even trying to do your own research, but I'm going to assume you simply don't know much about the setting so try to look things up before posting.