What I don't realize is how Waze, owned by Google, has more roads (like 50% more...

uptown · on Dec 20, 2017

This is their answer to how, though it doesn't answer the "why" question:

"The maps are completely separated. Waze and Google do share traffic info and reports.

Waze uses Google maps database for search (besides our own db ofcourse).

Sometimes Google maps data is imported into Waze. Streets, streetnames, parking lots.."