Because doing so would require them to either purchase licensed spectrum (way too expensive) or start a virtual carrier (an "MVNO") and then either provide thei

Because doing so would require them to either purchase licensed spectrum (way too expensive) or start a virtual carrier (an "MVNO") and then either provide their own SIM cards and hope that users can unlock their phones, or somehow enter into a roaming agreement with the big carriers. It's radically easier to just offer Wi-Fi. There are also new Wi-Fi advances designed to reduce the friction of finding and connecting to a Wi-Fi network. A modern Wi-Fi network is also capable of dramatically greater speeds.