From a design standpoint. Masterworking a weapon is a huge upgrade to its power. It generates orbs which is a buff to super recharging for the whole fireteam. Plus you get a significant bonus to one of your weapon stats. You want it to be rare, and something you only do with your most prized weapons. While infusion—as a leveling mechanic—-is one of the most mundane acts in the game. So mundane that every other rpg handles them with currencies that are freely farmable. By the time you add enough cores to the game to restore infusion to its proper role in the game, you’ll have people with thousands of cores and can masterwork every weapon they have. You make it into a trivial matter.