I haven't been following the issue, but:

The government believes mandating ATSC 3.0 in smartphones will help bring down network congestion in wireless networks.

So, over-the-air television uses broadcasting. You broadcast one signal, everyone in the area can receive it. You can have one viewer or a million viewers and it takes the same amount of bandwidth.

But if everyone is streaming video unicast, the way they typically would over the Internet from somewhere like YouTube -- which has the benefit of letting people watch whatever they want, whenever they want, independent of anyone else, you can't do that; you can't have a million viewers in a cell, or anything approaching that, because bandwidth consumption scales linearly with the number of viewers.

I know that the US emergency alert system uses broadcasting over the cell network, so there has to be at least limited support for broadcasting in the cell network, though. I dunno if cell providers use it for pushing out system updates to phones, but if they don't, I suspect that they should.

googles

https://www.ericsson.com/en/blog/2022/12/multicast-broadcast-group-communication

5G multicast-broadcast for group communication: Why it matters and how it works

Through 5G NR multicast-broadcast functionality, 5G networks can now be equipped to support efficient, reliable and scalable group communication services. Below, we explore the 3GPP technologies bringing high-performance connectivity to mission critical use cases.

It sounds like there is some kind of way to do broadcast/multicast within 5G, though.