Sure they can. For the very discerning eye, like yours or mine, we can see a difference. But most people either don't see it, or else don't care because the difference is very minor to them.
I can tell you my wife doesn't see a difference. The Xr is perfect for her.
Maybe if I pointed the differences out to her with the Plus next to it, she'd see it, but why bother doing that if she has no complaints?
I have a 6s and a friend has a 6 Plus. When I compared our phones, I could tell that his display was a bit more crisp than mine, so I understand the criticism that the XR has only 326 ppi and should have 401 ppi. I like the display of my 6s fine, but, sure, I'd rather have 401 ppi if I could. Still, it's not a big issue.
And, sure, I'd love to have an XS or XS Max, but 64GB is too little storage for me, and I refuse to part with $1150 for the 256GB model or even more for the XS Max. I have other financial priorities.
So I'm back to the XR. The display is a small annoyance, but when I'm checking the news or weather, listening to music or a podcast, texting or writing an email, I really don't think about it. I can buy a 128GB XR for $799, and its camera may top the X in areas where only one lens matters thanks to having, from what I've seen and read. The XR has the same A12 chip that the XS and XS Max have.
No, the display won't be as dazzling, but the primary reasons I've stayed with iPhones are that Apple has great customer service, the phones are quality and last, they get timely iOS updates, and, most importantly, they nicely integrate with my 10.5" iPad Pro and my still-going-strong mid-2012 retina 15" MBP. Is the XR priced too high? Probably, just as the XS and XS Max are, too, in my opinion.