Because they need to hide some of what they know about you. There have been cases where they (in this case target not google) knew a 14 year old was pregnant - but the ads for things she needed are sensitive. So the industry has learned everyone gets a few ads that don't apply just to give cover for those that do but someone doesn't admit to.
also some ads really are to everyone. You may not be in a place where you would think about a car, but car makers don't want you to forget you could just in case your situation changes. They also want you to think of some goods as luxuries so you are impressed when you see someone who does have it.
> Because they need to hide some of what they know about you.
My favorite example of this is in the desktop web version of Google Maps. If you search for some place and try to plot directions from “Your Location” it will prompt for the browser's Geolocation API and will refuse to give directions at all if you don't consent to the prompt: https://i.imgur.com/fIQswnD.png
This is despite the fact that it opens the map with a perfectly-centered and reasonably-sized window around my current location. I have never seen this fail when not using any sort of VPN that moves my GeoIP. They could totally give a reasonable mix–max time estimate based on that window just like the one they show for variable traffic.
also some ads really are to everyone. You may not be in a place where you would think about a car, but car makers don't want you to forget you could just in case your situation changes. They also want you to think of some goods as luxuries so you are impressed when you see someone who does have it.