I believe it is "easier" to use a computer that is out of date (so many PCs out there are still on Windows XP/7) as most of them can be perfectly functional offline, and even if they're online, an up-to-date browser is simply what most people would need.
It's quite different for a smartphone, where it is connected to the internet constantly. Sure, you can go offline, but then you just have a dumbphone. Most of the smart in the smartphone requires internet to be functional. Add on to the fact that you're tied to Safari on iOS, thus when it's no longer updated to be secure, there are risks considering the amount of personal data people have on their phones nowadays.
Personal example: I have no problem using my 2012 mac mini with Catalina, the soon to be obsolete os. I just need to use the latest Brave/Firefox browser and I will mostly remain secure. But for my iPhone 7+, I'm less comfortable being left on iOS15. I would be looking to get a discounted 13 mini later this year to ensure I have an up to date phone.