I get what you are saying and I agree what someone else posted, that video game writers are just going with what they know. If they are white they are likely to write white characters.
Yet I would point out that video games are not perportionate to the demographic you cited. And I have seen some charts that put the margin of white in america is 40-50% now. This is not properly demonstrated in media because 90%+ is white male, not 50%. Almost all movies/video games of yesterday and today have the same exact archtype white male leads. The number of video games that put a Black or hispanic(god forbid....an asian) main character on it are very slim. Mafia III, Dishonored 3, GTA San Andreas, Just Cause, Watch Dogs 2, Sleeping Dogs and.......I think that is about it. It gets really dicey when you bring in the fact that 50% of the population is female yet still..... 90%+ of the leads are male. I can probably name more than 100 games with character archtypes as the Previewer stated.
The few games that are experimenting with diversification are selling less(except in the case of games like GTA SA), even when in a anthology series. Why is this? The quality of the game itself could very well be different but at least part of that factor is thought to be white male gamers have a harder time identifying with characters that are not on some level similar to them, or at least the pennacle masculine archtype they are used to. This is often sited as a capitalistic reason on why games do not deviate too often from the traditional. Yet if this even is the case, I would state that black/hispanic/asian/women/etc have been consuming video games for a long time, combined make up 40-50% of the marketshare, yet have always just been expected to be able to find common denominator with characters that don't look anything like them.
And then people complain about anyone wanting to see a little more diversity, not even in race....just give us some variance from the gruff white male lead. Give me a bookwormy clean shaven guy with a normal voice. Sure, it feels pandering now but developers,fans and critics alike seek to make up for lost time.
Not sure what source says whites are only 40-50%, I have yet to see any official numbers that look like that. Usually, looking at official Census sources, it is somewhere between 55-65%.
Also, if they are catering towards American market, that means they are catering towards the main gamer demographic, not releasing games proportionally to our demographic base. If whites are the majority of Americans, then it makes sense that 90% of games (if that's true?) have white main characters, because that is the majority market. I am speaking from a business standpoint, I have no personal concerns over what race characters are and I don't really care one way or another (if a game is good, not inappropriate, has a good story, and is fun then I generally enjoy it).
Regarding females, while they have half of the population how much of the gaming population do they have? Again, I don't care what race or male/female they are.