Why are receptor-ligand complexes of polypeptide hormones internalized by endocytosis?

To prevent unwanted ligands/inhibitors from competing with the right ligand for the active site in the receptor.
another reason could be case dependent one. like for example if the outer environment is hydrophilic and the receptor or the outside of the receptor-ligand complexe hydrophobic then the complex will prefer staying away and thus internalized.
just think about the basic things and they will lead you to the reason

The answer varies a bit depending on the hormone. For some, the complex is internalized, the hormone is degraded, and the receptor is recycled to the cell surface. In this case, the purpose is to stop the signal once hormone is no longer present extracellularly.  In other cases, the receptor itself is also degraded, reducing the number of surface receptors and attenuating the signal, reducing the sensitivity of the cell to further exposure to the same hormone. In yet other cases, this process can facilitate post-translational modification of the receptors that desensitize them to further stimulation.

But the main purpose in general is to stop or attenuate the hormone signal.