Here's why:

* Rough Endoplasmic Reticulum (RER) is the site of protein synthesis for membrane proteins. The RER is studded with ribosomes, which translate mRNA into proteins.

* Smooth Endoplasmic Reticulum (SER) is primarily involved in lipid synthesis, detoxification, and calcium storage. It doesn't play a direct role in protein synthesis.

How membrane proteins are made:

1. The ribosomes attached to the RER translate mRNA, producing proteins that are destined for the cell membrane or for export.

2. As the protein is being synthesized, it enters the lumen of the RER.

3. The protein folds and undergoes modifications within the RER.

4. The protein is packaged into vesicles and transported to the Golgi apparatus for further processing and sorting.

5. Finally, the protein is delivered to the cell membrane or secreted outside the cell.

So, while both the RER and SER are components of the endomembrane system, they have distinct roles in protein synthesis and membrane protein production.