But that looks really bad, and to make it look good (as almost any even mid-budget video game looks) you need an art director who can deliver consistency. For a small game that art director might also just make all the art, so it ends up consistent that way, but games even from the C&C era are too big for that to be realistic, so now you've got several (and maybe hundreds of) volunteers making art and they need to obey that director's stylistic vision or the result is something that lay people can see is "wrong" but they aren't sure exactly why.

There's a little element of this in the programming side. One guy who stubbornly insists on calling loop variables 'x' another insists they should be named 'k' and three others who don't care can already get heated. Or maybe you've run into somebody who believes code should use character U+0009 for indentation instead of using U+0020 as god intended. But the players won't notice that part, whereas if the art isn't consistent that's very visible and detracts from the experience.

So the talk about open source might just be marketing fluff.