Part of the reason for that is that it's hard to be satisfied with relatively little when some others have so much more. (Ironically, some of these people long for a simpler, less materialistic past, but aren't content to just minimize their own amount of material goods; they'll only be happy if everyone else has less, also. That's the nature of worrying about relative status rather than focusing on absolute status.)
But we are absolutely better off than people in past generations, and if you don't realize that then you don't have an accurate historical perspective. And we're better off because capitalism makes it remunerative to find ways to make people better off.
I could address that in many ways. Clothes are much cheaper now than even when I was a child, making it easier to keep yourself and your family well-clothed (the winter coats I bought my children were roughly 1/5 the cost of the winter coats my mom bought me).
Housing costs are a common complaint, but part of that is regulation that makes it harder to build houses, artificially constraining supply, and part of it is the increasing size of houses - new houses have nearly doubled in size since 1950 even as the number of children per family has halved. The cost of housing per square foot hasn't changed much, but there's much more living space per person on average now.
Food takes a smaller share of a person's budget than in the past, meaning we literally can eat more for less work (yet some people fantasize about having a farm to feed themselves, guaranteeing they'll work harder for food).
But I think the most important change is in medical care. In the past several decades we have made amazing advances in both life-saving care and in care that improves the quality of life for countless people with illnesses or disabilities. And we are on the verge of cures unimaginable (except in science fiction) just a generation or two ago.
Without capitalism, those medical advances stop. In fact if we move to a moneyless society, as some of these folks want, they almost certainly reverse. Forget about preventing river blindness in Africa. Forget about preventing vitamin A deficiency in developing countries through the production of golden rice. Get used to increased rates of infant mortality and women dying in childbirth. Forget about helping paralyzed people walk again. Forget about ever more sophisticated artificial limbs.
Anti-capitalism pretends to be humanistic by pointing to the imperfections of capitalism and comparing them to an idealized non-capitalist society. But that's the nirvana fallacy. When you think seriously about what we'd lose, anti-capitalism's naive romanticism is revealed, and it's inadvertent inhumanity becomes clear.