Try doing that with a lot of parallax movement and DoF and it will look like a fest of weird blocky artifacts, because different depths of the image move at different speeds which means you would need to blur them into the dorection of the movement at different depths. Moving objects can become semi transparent at the edges which means generating that will involve knowing (or guessing) what is behind objects. This is non-trivial and very hard to solve in a general way.

The current best way to do this that I know of involves a ton of manual masking of elements at different z depths and the results will look still worse than if you had done it in camera from the start.

Nobody wants to have to do that to 40 minutes of a 90 minute movie unless there is a really good reason to do it.

That's surprisingly hard to do well, and is a very very very slow process.

Yes, increasing costs (artist- and technical) even more in post production.