It seems simple to implement, and it might lead to more engagement (for the platforms that care about that).
Instead, people rely on third-party tools to accomplish scheduling on Reddit and Instagram. Why is that? Is there some issue I’m not considering?
There’s a cost to maintaining a feature. Unless a certain percentage of users would actively use the feature and it would improve your KPIs more than another feature that could be implemented in the same period with the same maintenance cost, it’s not worth it and will never be prioritized.