Here's why:

* Thin: Cell membranes are extremely thin, typically just a few nanometers thick.

* Flexible: They need to be flexible to allow cells to change shape and move.

* Selectively permeable: Cell membranes control what enters and exits the cell, allowing some substances to pass through while blocking others. This is crucial for maintaining the cell's internal environment and carrying out its functions.

It's important to remember that the cell membrane is not actually a bag. It's a complex structure composed of a phospholipid bilayer with embedded proteins. However, the "thin, flexible, selectively permeable bag" analogy helps to understand its basic function.