Because they are really only $300 after finding them on sale or as a carrier promotion to switch and a trade in.

The are people who do pay full price for phones. Not many, but they do exist.

I pay full price and don't do trade in. My old phones I usually pass on to family members after I've verified that everything has been migrated.

When they go to my grandmother, it has the benefit of upsetting parts of the family, who get upset she gets something better than they have.