I personally think, too, that most FOSS games are either very badly made - especially when it goes to art and gamedesign - or are just a bad clone of a popular game. But I think the reason is somewhere else.
Most projects were initiated by programmers, who had a cool idea and wanted to make a game out of, so they digged out some webspace out of the internet and went onto the game. That leads most of the time to... nowhere. Two problems lie behind that failure:
1) Games are drawn, not coded.
2) Games get designed using a specific model.
FOSS world lacks artists in general. Especially talented and/or experienced artists. Only a few projects can count on the experience and talent of artists. Most media in FOSS projects are made by unexperienced people. I don't want to say that it is a bad thing when people want to start with digital art respectively game art - though said media is a keyfactor in the lack of "quality" in FOSS games imo.
So, why aren't artists getting attracted by such projects anyway? Linux and Open Source in general has having a boost of popularity, lots of new users, different kind of users. Where the heck are those artists?
In my opinion, said artists avoid using FOSS software - due to it's lack of quality as a tool for game development. My spotlight goes to a specific set of tools. For the Quake 3 engine. And actually for any other game or game engine using the Quake 3 BSP format (such as Nexuiz/Darkplaces, Transfusion, Warsow/QFusion, XreaL, etc...). Those builtin-tools are... well... ****, litterally. They are nothing compared to the tools of commercial engines (Source, Unreal, any other engine...). Buggy, circumstantial, bad code. Sure, the tools for Source aren't less buggy (Steam and stuff) or less circumstantial, but hey, they work. I as an artist wouldn't want to work with such an engine. And I don't.