Question: "Why is water leaking from my car door?"
Answer: Water is supposed to make its way through and drain out of the bottom of the door. There are two issues that can happen that can cause that water to leak inside your car. The first and most common is that the door membrane or splash barrier is compromised. Usually these are just a sheet of plastic which over time shrinks and pulls away from where it is supposed to be. Some cars have a metal barrier which is sealed in place, and the sealant can perish.
The other issues that can cause a leak is if the drainage holes are blocked, this can cause the door to fill with water and overflow. This doesn't happen very often, but is typically caused when a window gets broken. The window is replaced, but broken glass wasn't cleaned up from inside the door. As dirty rain water filters through the broken glass, over time it deposits dirt and grit until the holes are blocked.