Why do so many defend capitalism?

Because they benefit from it?

This is only part of it because the vast majority of people do not benefit from capitalism. Not anymore at least. Of those in the working class who defend capitalism I would say most are victim to the constant stream of propaganda that the media pumps out at the behest of the capital owning class. It’s something that starts from birth in most western nations and permeates into the narratives people are taught in school in a way that requires deliberate and consistent work to unlearn and break away from.