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?
Most social media platforms make money when you actively engage with them. Having scheduled posts removes that from them. So now they have less revenue and they have to spend more money maintaining a feature. As such there’s no incentive for them.