This is VERY debatable because statements that broad are almost always false. There is no need to have a cellular->IP->cellular bridge for 1:1 communication involving more servers, more service providers. If anyone wanted to they could implement at least the 1:1 signal protocol and probably even the messaging layer security protocol on top of SMS to get e2ee group communications.
Nobody wants to because cell providers sell SMS for horrendous prices compared to internet access.