It's like how the US got so far ahead with reliable universal twisted-pair phone lines, it took a lot longer here for people to really start having just a mobile phone compared to countries that were able to side-step their inadequate landline infrastructure with mobile phone services. WhatsApp largely gained prominence in countries that had much more cost effective data plans than SMS texting, making WhatsApp cheaper than SMS along with being available at all on Android (vs iMessage). In the US we had unlimited SMS messages included in plans before anything approaching that for data. So in the US SMS was already quite well entrenched in social circles by the time iMessage came out. And Apple knew that people wouldn't be so loyal to iMessage if they couldn't message with whatever non-trivial percentage of the social circle didn't have an iPhone so it inlucded SMS-fallback. There's plenty of WhatsApp and Signal users in the US at this point. But nobody cares enough to try and get everyone to switch to one platform that is available everywhere and doesn't have SMS fallback, so SMS is still the glue that holds all the messaging together in the US.
edit: I think I can safely say it's going to take the big 3 carriers in the US all agreeing to drop SMS (which pretty much will probably take the GSM standards body dropping SMS) for the US to truly move off of it.