Maybe think of it this way. Buying a PS5 game on disc lets you play on PS5. It doesn’t let you play the game on Series X or PC. In simplified terms, in the world of digital stores the license can be thought of like the physical disc. You buy a digital license to play a game on a specific platform (eg Playstation, XBox, Steam, Epic, Battle.net etc..), these licenses are tied to your platform account, only function on that specific platform and do not transfer across to other platforms. Just like a disc for Xbox won’t give you access to the game on Playstation. As the Destiny 2 expansions are sold via the platform holder digital stores they are not transferable to other platforms. On the other hand the season passes are considered to be in game purchases. Because they’re in game purchases and not bought directly from the platform store they are tied to your Bungie account and are therefore accessible on other platforms. While it may be technically possible to transfer / share a license across platforms each platform would need to share their customer data with one another which is something their customers would need to agree to. There would be technical difficulties and costs involved in doing this, allied to the potential for this reducing income means the various platform holders won’t do it. They each want their cut from the sale of the license, so in reality it’s not going to happen.

Could be obviously doable if bunji treated expansions like they do seasons. I know I know, I’m a freakin genius. 🤑

Something you need to know.... You don't own a game or dlc... you own the licences to access that content. Bungie is the only one that owns them. Game, dlc licences and in game currency are controlled by the platform provider you bought it from not by Bungie... and they don't like to share. Every time you buy a game, dlc or in game currency a percentage of the money you paid (around 30%) goes to the platform provider. This is how the providers make their money including Microsoft and Sony as they sell consoles at cost with any profits going to the retailer. Now you come along at at YOUR choice want to play on a different platform. That new platform does not have access to what you own on other platforms and in fact don't care. They just want their money before they allow you the licences you want on their platform. Bungie can not afford the cost of buying you licences for different platforms either as it would soon cost more money than you paid for the game. Playing on different platforms is your choice no one else's and Bungie allows it only because of player demand. As it is your choice to switch platforms it is considered that it is a "informed choice" and you are fully aware of the consequences of doing so the main one being you need to buy any content you want again.

very new account somehow want to play from steam to epic game ... are both on PC up next he will want to transfer from epic game to windows store ! also on PC