Because the problem domain, process and tooling is specific to your company and a senior person also needs time to learn them.

Plus, turning a junior into a senior developer isn't exactly a boolean switch, that is turned over night. It's a process and that junior developer will spend a lot of intermediate time between both states, in which he's still very useful, yet affordable since he's probably paid below senior level.

That's true. Presumably the junior developer is cheaper while they're junior, but if nobody's hiring them anymore then I guess they're not cheaper enough to compensate for needing to train them.

Maybe by creating a workplace where developers want to stay you could keep them around at slightly under market rate? Not everyone is looking to job hop every 6 months. But the amount below market rate that you seem to end up by staying in one job feels like it's unreasonably large.