I think it was largely due to their historically bleeding edge fabrication tech/node and market share. Until the recent resurgence of AMD and rise of ARM, Intel

I think it was largely due to their historically bleeding edge fabrication tech/node and market share. Until the recent resurgence of AMD and rise of ARM, Intel was able to sell out their inventory and keep their fabs at 100% capacity. But given the combination of losing the node advantage to TSMC and performance advantage to AMD & ARM, I'm guessing Intel has accumulated inventory and this poses a business risk. Opening up their fabs in the future is a way to mitigate this risk; essentially becoming both a chip seller and external manufacturer. The other reason is probably textbook monopolistic behavior: vertical integration means if you want top tier chips, there was no other option but Intel. Often overlooked is how closely correlated fabrication node is to chip performance, e.g. if Apple's M1 is fabbed at TSMC 7nm instead of 5nm, the performance wouldn't be so widely lauded.