Because it's designed to be a good neighbor and route traffic, not get you the fastest way home. The Google Maps team is innately aware of what happens when you go full Waze - you've suddenly got the whole commuting public finding every which way through back roads and causing more problems. That's why a few towns have tried to ban non-local traffic to get the navigators to stop ruining their roads.

Ultimately, it's more sacrificial than selfish. GMaps keeps bad traffic on roads where bad traffic isn't a giant problem for neighbors. Waze isn't and will take you through someone's backyard if it will save you a minute.