I think many platform did support RSS, or API, but at some time it was dropped. It is not that hard to provide RSS. It is not like it has to be implemented from scratch. It is just dropped by platforms. There must be reasons for dropping. One may argue it is not worth it, but even to drop functionality there needs to be a decision from management. The management always think about money. Is RSS or API monetized correctly? Free? No? Then we drop it, because data from algorithms serving user contents can be easily monetized. Just follow the money, the oldest truth.
That said, this seems quaint in the modern era where trillion-dollar tech companies are doing that to publishers now, too.
Though some seem to go out of their way to make RSS impossible.