If I remember correctly, fats are very similar to plastics in molecular structure. The oils in pasta sauces get stained red, and in turn the oils like to hang on to the plastic, making them stained. It gets tricky to thoroughly wash all of the oils off the plastic because they want to stick together. One of the tricks I’ve heard to prevent staining is to spray plastic containers with regular non-stick pan spray before adding the pasta, so the clear non-stick spray oils sticks to the plastic before the stained pasta oils do.

This is also why you can wash a plastic container and sometimes it still feels a little oily or takes finger prints easily.