While there can be a great deal of pressure pushing the oil up the well, there is also resistance to that flow up a narrow pipe (perhaps 4 inch pipe). The balance of forces can be very close but the momentum can keep the flow going. In some fluids, there can be methyl hydrates (or wax) that can cause problems with maintaining flow. There can also be particles that are moving in the fluid (sand), slowing or stopping the flow can cause these particles to settle, preventing future fluid movements. There is a temperature and pressure gradient which can allow dissolved gas to bubble out the fluid as it ascends the well which can lead to added complexity. And we haven’t yet discussed oil recovery methods like gas/water injection or electrical sub surface pumps. Getting a well restarted after shutdown, may not be possible. I worked with some petroleum engineers, getting the best from your wells can be a very complex simulation problem followed by expensive engineering solution.

No, it's a matter of the complicated and only partly known geology underground. See other answer.

Another way of looking at it is: You've drilled holes to make the oil flow. There are kilometers of rock on top of the oil layer, pushing down, pushing the oil towards the holes you've drilled. You can close the end of the hole, but you can't make that weight go away. The system is going to find a new equilibrium. The new equilibrium is not one you've tried to optimise for maximum total oil production.

It’s simply not true that you don’t know if the oil will flow again. It will. The subsurface pressure and fluid behavior is well understood.

One of the only scenarios that might lead to wells failing to flow when reopened would in wells that were freshly tracked without flowing back the frack water. In this case the frack water will soak into and damage the clay-like minerals in the rock and damage the permeability of the rock.