Here's why:

* Rough ER: The rough ER is studded with ribosomes, which are the sites of protein synthesis. Proteins made by ribosomes on the rough ER are then transported into the ER lumen (the space inside the ER) for folding, modification, and further transport.

* Smooth ER: While the smooth ER doesn't directly synthesize proteins, it plays a role in transporting them to other parts of the cell. It also assists in modifying and packaging proteins for their final destination.

So, while the ER isn't the only organelle involved in protein transport (the Golgi apparatus and vesicles also play crucial roles), it is the primary site for protein transport within the cell.