Because it is difficult to send high volume, repetitive messages if you have to do them by hand.
However, I’d note that the terms of service of my domestic mobile account, not in the USA, say they they can block me if I send the same message to multiple people, and they do, apparently, monitor for this. You can’t even implement a neighbourhood watch type alert system.
Whilst this indicates that some control can be applied without the need for a registration system, it does show that even personal mobiles can have restrictions on their use of SMS, outside the USA.