Possibly to make money. Another convenience tax.
As @meltam6554 outlined, one would be better buying a entry level android tablets with inbuilt GPS, and installing one of the free GPS apps.
As outlined above, we use Mapfactor GPS and have found it very good, and better coverage in remote areas than in car GPS mapping systems.
The disadvantage is the tablet isn't built into the car meaning it needs to be held by someone or placed somewhere in the vehicle. Also one needs to remember to take the tablet when travelling.
The advantage is one can use the tablet for connecting to WIFI when travelling/in the home, storing ones own music collection (if additional storage is available) for playing in vehicle if one has bluetooth or AUX jack and other such use.