Maybe so it doesn't accidentally eat through someone's data? Not everyone has an unlimited plan. I'm a very light phone user (when I'm away from home) and only have a 2 GB/month plan through Cricket. On the other hand, perhaps it's a mandate by carriers so unlimited users don't use excessive data with updates. I imagine thousands or millions of people updating via LTE/5G would quickly clog the airways.
Maybe so it doesn't accidentally eat through someone's data? Not everyone has an unlimited plan. I'm a very light phone user (when I'm away from home) and only have a 2 GB/month plan through Cricket. On the other hand, perhaps it's a mandate by carriers so unlimited users don't waste data with updates.