because SMS isn’t data.

SMS is a 2G standard. Those networks were very limited in capacity compared to today, so SMS basically grew up with limited capacity and it’s a legacy hold over, and/or some carriers for a long time were still carrying SMS/phone over the older standard towers.

2G and 3G towers were only very recently retired, like 2022-2024, at least in the USA. In other parts of the world they are still active.