Why was the highest magistrate in Rome called "Pontifex" (= bridge maker)?

For the same reason the Pope is called the Pontifex Maximus, they work as god, gods', or goddesses' agent here on earth, thus bridging the gap between heaven and earth. They're interpreting what the divine beings want for us here on earth. "The term pontifex was also translated to "bridge builder" as the position acted as a "bridge" between God and men."

https://study.com/academy/lesson/pontifex-maximus-overview-history.html