if someone makes a one word comment, they're unlikely to want to write you a research paper justifying it. but i'll give it a li'l go.

the whole core concept of capitalism is that some few people 'own' the ovens, and they get to tell other people to bake cookies, and even though these cookie-bakers bake all of the cookies, the oven-owners get to keep most of them, and the cookie bakers only get a small portion. also there aren't any ovens that aren't 'owned' by an oven-owner, so if you wanna eat cookies, you have to give most of the cookies you bake to an oven-owner. and this model is extended to everything.

i don't know that this allegory will mean anything to you, and maybe you think of something different when you hear 'capitalism' (maybe you think of 'markets', or 'money', or, i don't know, just seeing benefits from ones own hard work or something), but this is basically the usual meaning of capitalism, and probably what most people who oppose it, myself included, are talking about. there are a lot of other criticisms, of course, but that core scammy unfairness of it, i think, is most absurd.