Because not everyone can stomach the idea of living in a country where other people live in abundance while others, for whatever reason, will starve, freeze to death, or die from preventable illnesses because of lack of access to food, shelter, and medicine.
Ideally, I wouldn't have to pay more to cover everyone else because everyone else would have received the same access to social support, loving parents, high quality education, physical safety, excellent healthcare, clean water and nutritious food, and many other resources and services (and just dumb luck of not having any congenital diseases or traumatic accidents) I received from the first day I was born, and which made me into the highly capable and functional adult that I am today. That's not the reality, and although it's not my fault that other people didn't have the same opportunities I did, it is still my responsibility to acknowledge that many of the advantages I've had in life are for reasons that have nothing to do with merit or character.
That responsibility is why I think I should pay more to cover everyone else. Plus, I prefer to live in a society where people care about each other regardless of their individual ability to generate money. In part because I think it's ethical, but also in part because of self-preservation - I won't always be able-bodied enough to participate in the workforce, and hopefully I will have saved enough money when that moment comes, but there's a lot of uncertainty around that, and I would prefer to work towards building a society in which I do not have to live with that uncertainty (which means no one has to live with that uncertainty). (I do currently have disability insurance, which mitigates some of the uncertainty, but isn't a complete solution.)
That doesn't mean I think everything should be free or that all occupations should make the same salary. But I would prefer that instead of punishing people for not working "hard enough" by denying them access to food, shelter, and medicine, we reward people for hard work/productivity/innovation/whatever it is you value in others by giving them access to luxuries / non-essential services / more lucrative entertainment / more conveniences/ more choices in acquiring material goods, etc. Obviously, there is a lot of subjectivity in terms of what is essential versus non-essential, especially as people get used to a certain standard of living (e.g. refrigeration, phone and internet service, air conditioning, transportation), but rather than let prefect be the enemy of good, we can at least ensure people have access to some basic things and adjust as we go along. We don't have to get it completely right, but we can certainly do a much better job than what we're doing now.
