Apple has famously made its strategy to use iMessage to enforce exclusivity. If you want to reach everyone, it's not iMessage. And Whatsapp in the US is worse, closer to 1/3.
WhatsApp does not solve any problems for the typical American user. Most Americans don't install WhatsApp unless they spend a lot of time overseas some place where it is required to do anything. Even international group chats seem to be more Discord-based in recent years.
I'm Irish and travel back and forth a lot. First, not everyone has iPhones, Android has 40% of the market.
Older generations use Facebook to manage their clubs. I'm increasingly seeing Whatsapp and occasionally Signal for younger and more tech-savvy social circles. Facebook is non-existent in sub 35 year olds. Its just taking longer to switch over (or away from) Facebook given how tech savvy older folks are here compared to Ireland.
* pretty early on the vast majority of phone plans started bundling unlimited text messaging, which IIRC was a big motivator for using messaging apps abroad
* because of the vast scale of the country, domestic coverage results in no roaming for the places Americans spend most of their time, unlike in Europe where there are multitudes of countries you'd be passing in a one to two hour flight. Roaming charges in the EU were only abolished in 2022, late enough that everybody has settled on apps as the best way to manage that now.
* many American plans extend unlimited messaging to Canada and Mexico, the two likeliest places that Americans would go to abroad
Or just iMessage with fallback to SMS for those not on iPhones. Unlike most of the rest of the world, iPhones dominate in the USA.