Imagine if FD just added atmospheres and oceans without all the attending things they would require. I'm sure certain forum members would start threads complaining about how poorly they were implemented. How the flight model doesn't change when you are in a thick atmosphere, that sort of thing.
How long would it take FD to do what is required to add real atmospheres and oceans? No idea... let's guess at 1 year. So, that would be Odyssey delayed for a year. You want Odyssey delayed a year? Or better they get Odyssey out when they plan, and deal with atmospheres and oceans later?
I doubt it takes 1 year to expand the flight model for atmospherics. Well ideally they'd be working on that since 2019, but I guess they don't have the manpower or budget to do it for this expansion.