There is historic reason for this : all carriers and phone makers see messaging experience as critical and want to control it themselves. Google didn't have eno

There is historic reason for this : all carriers and phone makers see messaging experience as critical and want to control it themselves. Google didn't have enough pull to make messaging mandatory part of default google apps bundle. Then iMessage and WhatsApp happened and Google desperately tries to rectify the situation. See Hangouts SMS integration. Latest two-prong attempt is Allo and RCS. Allo failed. The reason RCS kinda works out is that carriers can't combat iMessage alone anymore. RCS was conceived as asymmetrical response to iMessage which aligns both Google's and carriers' interests. Many carries don't even try to implement it and rely on Google's default client implementation and route through default google servers.