Not OP but I presume a major reason is that the LG display (5k Ultrafine) only has a TB3 video input which means virtually all GPU cards can't drive it. I ended up with a HP Z27Q 5K display for my setup but would have much preferred the LG if it had more input options.

He said he wished Apple would sell a stand-alone display. If Apple sold one, what are the chances that it wouldn’t also be TB3 only?

BTW, as far as I can tell, HP no longer sells the model you bought.