To give a quick example, consider the following: I plant an apple tree, wait for it to grow and then pick 20 apples from it. My neighbor only has two apples that he bought at a grocery store, since he doesn't have his own apple tree. No one stopped him from planting one, he simply chose not to, which is entirely fine, since no one has an obligation to grow crops. However, would it be okay for him to forcibly take nine apples from me so that we have the same amount? Would you call this "fairness"? I wouldn't; I would call it theft.
Sorry to be the bearer of bad news, but having less than someone else doesn't make you entitled to what that person has. You don't get to knock on a millionaire's door and demand $50,000 so that you can pay off your medical bills or tuition. This sounds rather obvious, so why do I bring it up? It's not as if poorer people are personally forcing the rich to give them money. Except some of them are, when they vote for politicians to pass laws that force the rich to pay taxes. There's not much of a difference between stealing for yourself, or asking someone to steal on your behalf. It's still theft. Again, calling it "paying their fair share" or a "wealth tax" is just a nicer way of referring to theft. It is the seizure of money that doesn't belong to whoever is forcibly taking it. That money doesn't belong to the government, and it doesn't belong to you, so it's not okay for the government to take it, and it is not okay for you to receive it from the government.