The main challenge for building a game is the content - the art, level design, enemy design, terrain, story, etc. The game engines and coding tend to be (a) more portable between games and (b) have more people interested in contributing relative to the need.
I was part of a game design group in college, as an extra curricular. We had tremendous unmet needs for art and sound design, which really limited the games we could make - maybe one or two game the 6-12 projects each year actually had the skills they needed. Part of that was probably the group running out of ACM and thus just having better ability to attract CS than art types, but I got the sense it was partly the type of people interested in building games for fun.