I think by "these services" they mean apps like Doordash, not delivery as a concept
I know much of the answer is a mix of private equityb and an overload of debt taken from insane evaluations.
1. Pizza travels very very very well
2. Pizza is pretty cheap to make
3. Wages (and costs of transportation) were lower 20 years ago.
More generally, delivery as a model can work, but not when you have an organisation of really expensive engineers/salespeople working on a frontend to it.