Why does the plasma membrane exists in all living things?

Answer 1

We cannot know for an absolute fact that it does exist in all living things. To know this, we would have to test every living thing. That is impossible.

Of course, maybe we can say that it exists in all things that have so far been observed. Its existence may reflect the common need for all cells to separate their internal environment from their external environment.

Additionally, all cells need to evaluate and respond to their external environment. These are critical functions (separation / protection, sense, response / adaptation) of the plasma membrane.

Answer 2

It regulates what comes in and what goes out of cells. It was the thing that allowed microorganisms to live in the prehistoric seas. It allowed prehistoric organisms to survive the UV rays given off by the sun. SO, only the organisms that evolved plasma membranes were able to live and pass on their genes to later generations.