TL;DR: Apple dominates the US smartphone market, but EU regulations may offer Android a chance for resurgence by enforcing messaging interoperability and standardizing hardware features.
Are WhatsApp, Telegram, Discord, Signal, and such blocked in the US?
Of course they're not blocked.
People just default to the app that comes pre-installed with their phone and sits right there on the first screen, because it's marginally easier than picking a third party app in the App Store, installing it, and creating an account.
It's the exact same argument that Microsoft made when they bundled Internet Explorer with their OS.