: Here's a quick run down of how: You have a bunch of skins and stuff, right?
: Okay, take those skins, and now add requirements to unlock them. You want
: a special DMR skin? You have to get ten headshot kills, or twenty, or X
: number. You want a shotgun skin? Twenty one-shot shotgun kills. Or even
: better, make it tied to playing objectives. Hold the ball for a total of
: 100 seconds. Hold the hill for a total of 200 seconds. Capture 5 flags.

Probably because that would be detrimental to the game. If you play Destiny, you know there's an event called Iron Banner. You get special bounties worth a lot of rep everyday the event is active. One of them is to get 10 melee kills. It's use it or lose it. The bounties reset every day. This ends up meaning that often when playing with random people, you see them sometimes just running around trying to melee stuff and end up dying.

When it's single player and you are on your own, that's one thing, but if you are costing your team because you want to grind out some bounty, then you are impacting the experience of others. Destiny is way more fun with a team of DBOers, but sometimes I play during the day when nobody is on and have to get matched with randoms.