During this generation of consoles, there's been a huge amount of games featuring space marines. According to Gears of War designer Cliff Bleszinski, technology is to blame.

"In this generation the translucent effects are very hard to pull off," Bleszinski told Develop. "That’s why everybody sees a lot of these bald characters and wonder why games feature so many fucking space marines. Well, the tech is good at showing off amour and it’s not that good at doing hair. That’s why we have bald space marines. There’s your answer people."

Well, that might explain the visual similarity between games' protagonists. It doesn't explain why they all fight aliens and say shit like "Stay frosty!" or "Can't wait to see your mom again, Sanchez!" There's a lot of companies that still don't place a premium on story-telling and just ape what's worked in other games.