They're not the same market. The XDR is a very, very niche product - I expect that the main group of people that will use them will be those doing the final grade during video/TV production. A lot of post-production houses I've visited still get by with aluminium Cinema Displays from ~15 years ago for their editors, producers etc. Most people along the production chain just don't need (/they can't afford) top end specs on a display when a basic model will suffice. This may differ in film, but I doubt it.It's not difficult at all. I think the reason Apple is not offering is that it will cannibalize the XDR sales. When Apple announced it @wwdc many workstations had as much as three XDRs on it. If you are a Pro on specific segments, you will only need one super-duper reference display if Apple had a 5K ($1500) display selling alongside the XDR. I can easily see a lot of people buying two 5Ks and one XDR just as one example.
A 5K display has a far greater market and I don't know why Apple aren't interested in it. It won't cannibalise XDR sales, as quite frankly if you are buying the XDR when you could make-do with a bog standard 5K panel from an iMac, you probably have more money than sense. The LG display is fine, but a 8-year old Thunderbolt Display still has it beat in terms of looks and build quality.