Question

Why do civilizations borrow elements from earlier civilizations?

Answer 1

Civilizations borrow elements from earlier civilizations because it takes less effort to borrow a pre-existing idea than to come up with a new one. And some ideas--such as systems of writing, agriculture, etc.--were so helpful that to ignore them would have been foolish. 

Answer 2

The fundamental civilizations developed urbanization and complexity without external influence, and without building on a preexisting civilization, although not all developed at the same time

Many later civilizations borrowed elements from them, built, or incorporated, through conquest, into other civilizations.

It was typical that politics and religion were strongly connected, thanks to this, the more people shared the same set of ideas, the people who did not know each other could find common ground, and build trust and mutual respect.