Because everyone seems to have a smartphone with a data connection. Roaming inside the EU is free nowadays, and an internal modem would add to the cost. (monthly M2M subscriptions)
The older Sim-GPS'es as you state, have become incompatible or are becoming incompatible or no longer supported because they only support 2G data. When tomtom creates GPS devices, they use a single platform, with 90% the same internals. If they can't guarantee 5-10y of compatibility with the cellular networks, there is no use of including such a component.