Doordash charges restaurants a percentage of the gross from the sale. Rather than eat this cost, restaurants are encouraged but not forced to add a markup on the prices they give Doordash (or insert your favorite third party delivery app here). They all do it.

If you order from a store's own website though, Doordash (I don't know if other third parties do this) did not "find" or create the business/order... they are really only handling the delivery portion.

In this instance, they still have some fees but do not take the large percentage, as that is a finder or broker fee. They aren't bringing the restaurant the business, it's the other way around.

Thus, restaurants can use their normal pricing. If you can find the places near you doing this, it's a much better deal than using Doordash normally.