Also, the default launcher happens to be something heavily customized by each OEM (likely extending the AOSP launcher which will never have ads or more typically extending a shared base launcher[0]), and I think routing ads through OEMs would be both a technical and logistical impossibility.
The phone space is such a competitive market, and the cost of switching (launchers/to a different Android phone/to iOS) is so low, and the risk to future sales so high, I really doubt you'll see ads on the launcher.
[0] https://cs.android.com/android/platform/superproject/+/maste...