Most farms are not owned by farmers, most housing is not owned by tenants, most airplanes are not owned by airlines, etc., etc..
Vertical integration can have benefits, but it isn't necessary and has drawbacks. Even when vertically integrated, regulations are often written under the assumption that everything is leased, not owned, so compliance is easier if you own a company that owns an asset, instead of owning that asset directly.
For example, if two people carpool in a car that one of them owns, instead of hiring a taxi, they'll usually split the costs of the fuel and the wear on the car. On the other hand, if they were flying in a small airplane owned by one of them, it's illegal to split the costs of wear on the airplane, unless its a rental or air taxi. Because of this, and other similar effects of FAA regulations, many small airplane owners own a company that owns the airplane, instead of owning it outright, and rent the airplane from themselves, whenever they use it.
Vertical integration can have benefits, but it isn't necessary and has drawbacks. Even when vertically integrated, regulations are often written under the assumption that everything is leased, not owned, so compliance is easier if you own a company that owns an asset, instead of owning that asset directly.
For example, if two people carpool in a car that one of them owns, instead of hiring a taxi, they'll usually split the costs of the fuel and the wear on the car. On the other hand, if they were flying in a small airplane owned by one of them, it's illegal to split the costs of wear on the airplane, unless its a rental or air taxi. Because of this, and other similar effects of FAA regulations, many small airplane owners own a company that owns the airplane, instead of owning it outright, and rent the airplane from themselves, whenever they use it.