The reason is that you're not paying for DLC licenses you already own. If you purchase the Microsoft Xbox One Destiny 2 DLC license, that's a different license from the Valve Steam Destiny 2 DLC license. It's two different companies that each sell access to their own platform's licenses. Just because the main name of the license, such as "Forsaken" is the same on both platforms, a lot of people assume it's the same license when it's not. It's somewhat like purchasing a ticket from Theater Company A to watch a movie, then going to Theater Company B and trying to use Company A's ticket to watch the movie. You didn't pay Company B for their ticket, and they won't accept your purchase from Company A even though the movie's the same; it's not Company A's showing, it's Company B's showing; the movie is the same, but the theater/seating/location/etc are different, so it's not the same movie experience. You're wanting access to that company's showing of the movie and haven't actually paid them for access. On top of that, each company wants paid for their license access. Purchasing something from Microsoft doesn't give Valve any money whatsoever, and Valve wants paid for their license access on Steam. Microsoft won't personally pay Valve for you to access to their version of the license. If you have an issue with the inability to redeem equivalent licenses across platforms, you'll need to direct your feedback to the platform companys: Microsoft, Valve, Sony, and Google. Bungie does not sell, grant, or handle the licenses; they're entirely handled, granted, removed, saved, etc from the platform companies themselves. Bungie and other retailers may sell keys that redeem licenses from the companies, but they don't handle the actual licensing.