Even many people familiar with pocket computers use the iOS Mail client. I was an email user over a decade before Gmail existed, and I used mutt for most of those years. I happily use the iOS email client because of the OS integration, clean UI, well-designed interactions, and decent support for wherever my email happens to live that year. I think mostly it’s because I’ve dicked around with computers enough in my life, and I’ve no interest in doing so with my phone, too.