If they made Google Maps better, users would replace Waze with it and their data collection would decrease. Since an app needs to run in the background to collect speed and location data, moving that feature to Google Maps would be a negative to most users.
Maybe at some point in the future there will be a universal app.
I think Google purchased Waze as a way to get up-to-date traffic information. Beyond that I don't think there is much integration between WAZE and Google Maps. Waze still uses community built maps for instance.
Maybe at some point in the future there will be a universal app.