There are plenty of reasons.
A robust ecosystem Privacy protection features and measures that are given first rate support - at least as far as you can say about a closed source platform Simplicity
I have years of experience with iPhones and flagship Galaxy phones since the 5.
I’m settled on the iPhone for now because those blue bubbles make keeping in contact with family easy. Sure you could use WhatsApp but if it’s not already being used, most people won’t want to download another app just to communicate. They’d rather use the built in texting app.
It also syncs well with my watch that I use for fitness tracking, and my Mac Studio that I use for professional work.