Here's why:

* The cell membrane acts as a barrier, controlling what enters and exits the cell. This is essential for all cells to maintain their internal environment and function properly.

* All cells need to interact with their environment, whether it's to obtain nutrients, eliminate waste, or communicate with other cells. The cell membrane is the interface for these interactions.

* Even prokaryotic cells, which are simpler and lack many of the organelles found in eukaryotic cells, still have a cell membrane.

While other structures like ribosomes are also present in most cells, the cell membrane is the only one found universally in all cells.