Why is delivered pizza often oily?
I have had a pizza with a drizzle of olive oil on top (in addition to some basil, pesto, and something else). As best I remember, it did not end up looking, feeling, or tasting particularly "oily", although it was visually apparent (olive-green lines).

However, that was a high-end pizzeria; I've never seen oil on a delivered pizza, at least not any that was intentionally added as a separate ingredient. Oil that is orange or yellow comes out of the cheese (and salami or other fatty ingredients) during cooking. Inexpensive ingredients often have much more grease than higher-quality alternatives.