[johnnyanmac]

>Why can’t we make popular open source video games

Nothing is stopping anyone and there are a few. Nothing as big as COD but they exist. But I imagine the result will be similar to how traditional sports work. There will be one big definitive organization for a sport (let's say COD or this example), a few large minor leagues (some knockoff CODs that are fun but nowhere near as popular), and then some small pickup games in a field (the indie games some single dev makes that no one will find).

As a complete tangent, the Touhou series is famously loose in its license and many fans and companies alike have utilized the IP to make games and every other kind of media.