It is mostly marketing and perception, along with having the most powerful card every generation, and features that AMD doesn't have or can't match, like Ray Tracing and DLSS.
That said, I own an AMD card (6600) and usually buy AMD and don't get why the masses don't switch. I bet a lot of people haven't even tried AMD. I used to be an AMD fanboy when I was young, but switched on a lark just to see how the other side was, and never switched back.
AMD was basically just as good (still is) but is cheaper too. I blame AMD's shitty marketing (it's generally been between okay to bad / cringe) and idiot customers who only want AMD to do better to lower Nvidia prices, but are too stupid to realize that if they bought AMD, not only would it lower Nvidia prices, it would also give AMD the funding necessary to improve their product, which meant they might not even have to "settle" for AMD anymore.