Why does it take so long to build aircraft?

There are numerous age sensitive materials used to build and maintain aircraft. Rubber tires, seals, hoses and gaskets, adhesives, chemicals used to make components like reinforced/composite structures (like fiberglass or carbon fiber reenforced plastic). Probably paint (it takes a lot for a large transport).

Don't forget that some materials are subject to aging while the aircraft is in operation and that and for each individual component that clock starts ticking well before the aircraft is delivered to the customer.