Why haven’t Manaphy and Phione been added to Go even though Sinnoh has been in the game for years

Seems very strange these Mythicals still haven’t been added, we had all Sinnoh Mythicals for ages so why weren’t these added for a Go Fest or Season? Maybe bc they haven’t figured out how to add Phione since you obtain it by breeding in MS. I feel like these should be in the game already but maybe there is a reason why they haven’t been added yet, I imagine if Manaphy is added it will be locked to a special egg like MS that will require a lot of KM to Hatch, but the what about Phione?