The geometric waveguide technology used in the Display glasses is quite complicated and costly, the birdbath optic that Xreal uses is stupid, so to speak, and various holographic solutions used by Even Realities and likes is not so spectacular in light loss(IIUC the fancy term is "quantum efficiency"), as well as multi color feasibility(needs 3 layers with separate image sources).
Prior to now we didn't know if Meta will be doing a Display 2 or Quest 4, or if they plan to do something else. This is that something else.