Here's why:

* Plasma membrane: This is the outer layer of the cell, acting as a barrier and gatekeeper. Receptor proteins need to be positioned here to detect signals from the outside environment (like hormones, neurotransmitters, or other molecules).

* Intracellular: Some receptors are located inside the cell, typically in the cytoplasm or nucleus. These receptors bind to signaling molecules that can cross the plasma membrane.

So, while some receptors are intracellular, the majority of receptor proteins are found embedded within the plasma membrane, allowing them to interact with molecules outside the cell.