The airlines are tenants. They don't own the facilities. They pay rent and landing fees to the airport authority.

Major airport projects are a joint effort between the airport authority, local & federal government, and the airlines. If the airport and government agencies are not ready to fund an improvement, the airlines can't do them on their own. Similarly, the airline has to be prepared to pay a portion of the costs plus increased fees to support the project. It is difficult to get everyone to agree and all of the funds allocated. It's no surprise that these projects take decades.