Why is delivered pizza often oily? Forgive the heresy, but I've recently had some pizza delivered. I noticed it was quite oily, and I have seen and heard horror stories of pizza basically drowning in oil.

This does not happen when I make pizza myself, even with salami on top the amount of oil is small, and certainly not soaking through and through.

Is the oil added intentionally? Some (e.g. Asian) delivered foods are put in oil to keep them hot, but that does not seem to be a viable option for pizzas.
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.