The DLCs are sold through the platform providers and are locked to the platform account. The platform provider gets a cut of each sale. Therefore if you buy on Steam but also want to play on Epic then you need to buy the content on both platforms. Seasons/Episodes are different because they’re in game purchases so are tied to the Bungie account, which makes them accessible across platforms via cross save/play. My advice would be to stick to a specific platform, eg Steam as keys for the DLC are available from legit places like cdkeys.com at greatly reduced cost.