That is because not every device out there spanning a wide list of them has been upgraded to actually be able to support 4g and 5g systems. As expected, the American carrier is among the first to do so in the states with T-Mobile and Verizon following later this year. As Axios notes, by AT&T ending support for 3G it won’t just be phones that will be hit.
3G still powers certain types of home devices; car management systems, roadside assistance devices, and a host of others.