Because it's a needless and pointless addition to the game. Not to mention you're just adding even MORE things to collect (currencies, essentially) when one of the larger complaints we've had in the past is too much of that. There's SO many games that offer that. This one absolutely does not need to, nor do I think it would benefit from it at all. The very last thing I ever want to see in Destiny is a skill screen like Runescape. Edit to add: We did, at one point, have to go out and collect planetary materials to upgrade guns. They abandoned that because the playerbase did not like it.