Apple has certain set profit margins. In order to hit a certain cost, yes, they offer various quality of components - for example, MacBook Air has a lower quality screen than a MacBook Pro. Likewise, even though both are called "Liquid Retina", the iPad Pro 2018 has a better screen than iPhone XR.
With that said, XR has a teriffic screen and you're just wrong about the importance of resolution. You know, if you said something like "why doesn't the XR have the same contrast/color accuracy of other Apple LCD screens, like the one on iPad Pro" - I would still think you were overreacting a bit, but the statement would at least be based on something that really counts.
If you want to argue Apple could've put a more expensive screen in the XR, feel free to do so. If you want to argue you can get a better product for less money - I'm not going to go into that discussion. It's old as Apple itself. Personally, I don't if they could've added a more expensive screen and kept the price point.
But even if you are correct, even if they should've added a better screen - increasing the resolution would not be the way to do it (yet, it would've made for better marketing). There are more important factors than resolution. Given the choice - iPad Pro level contrast and accuracy would make for a better screen then 400ppi resolution, and yet, no one mentions that, only that it's a lower res then they "expect".
In fact, a 1080p screen of average quality is actually CHEAPER than a great 720p screen. If Apple wanted to "reduce cost", they could've put a 1080p screen and cheap out on other parameters, like many Android manufacturers do. I think someone already posted this video, but here it is again, it illustrates my point well.