As a player? Or just as the father of modern chess strategy?
I have never heard anyone put him on their short list of greats, yet to do what he did would take supreme "greatness", plus he lost his title when he was almost age 60 years to a dominating 26-year-old genius named Lasker who would reign for nearly 3 decades and growing more insane.
Steinitz was the best player in the world from 1866-94 (longer than Lasker but without an official title) and he completely revolutioned the way chess is played, and he experimented OTB (which may have led to theoretically suboptimal play, but at least he had spheres), so why isn't he considered as one of the greatest players ever of all time?
I guess because he didn't play Morphy, but that wasn't his fault, so you really can't use that against him, much the way you can't penalize Karpov for not playing Fischer.