when I say linear perspective was rediscovered I don't mean the very idea of it, which is obvious to anyone with eyes, but how to make it happen properly in art.

the idea of 3D linear perspective is obvious, but its actual execution on 2-dimensions requires dexterity, planning, and an abundance of materials we now take for granted. Early medieval Europe had an economic scarcity of artistic materials, and a lack of erasable media (think pencils) leading to an inability to erase and redo art at ease. The process of making parchment from animal hides was intensive prior to the mass manufacture of wood-based paper. There was also a lack of widespread artistic education on how precisely to achieve linear perspective using vanishing points, coupled with artistic conventions that deliberately prioritized other things over realism. Everyone understands that things get smaller the more distant they are, but they didn't draw things that way for cultural reasons. The earliest art paintings and drawings typically sized many objects and characters hierarchically according to their spiritual or thematic importance, not their distance from the viewer, and did not use foreshortening.