I suppose because collections is about being able to re acquire items with rolls you've already had and not items with rolls you haven't had before. They probably could log the code for each individual roll of an item that you have had before but I imagine opening collection's would take considerably longer. I do think that it should be possible to re roll one aspect of an item already in your inventory. Like being able to re roll the masterwork or one perk for a cost of 10 cores per roll. Once a masterwork or perk is rolled only that particular perk or masterwork can be re rolled and nothing else on that item.