Because of capitalism I think, you can't just appropriate someone's profits, that's not capitalism.
There are two basic ideas to solve this though, you can put more and more taxes on the capital owning class to "make them pay their fair share" for, from what I gather the humanists want, an unlimited number of economic migrants because it's the human thing to do. Now an important thing to note, might be Trump's comments in the second debate, that while everyone is going on about how he legally avoided paying maximal amounts of taxes (which I would do as well, I think most everyone pays as little tax as they legally can), Hillary's donors also use those loopholes. It takes a lot more research into the respective tax plans to see if "making the rich pay their fair share" is a campaign line or an actual plan, so that takes checking.
I gather that the arguments against this are that if people can't be rich in the USA, they will go be rich somewhere else. The USA, from Trump's rhetoric, I haven't checked this, has some of the higher corporate tax rates in the world, and seeing as how manufacturing has already spread outside the USA, is it really crazy to assume the capital will as well (especially when selling the goods to America is facilitated by free trade deals that are in place, making it less costly to move your business to another country).
This is the Democratic party tack to deal with wealth inequality.
The second plan is to cause a shortage of labor, which would increase it's value, which would make wages go up. The common ways to do this would be: to make it expensive to import goods into America (which lessens the effect of labor in foreign factories which export into the USA) by getting rid of free trade agreements; and to lower the number of people in the USA competing for that labor. We attempt to do this by having an immigration procedure, with quotas, so that we can accept as immigrants into our labor force the number of people needed at various skill levels (this would be why it's easier for a doctor to get in than a farmer, say). Trump's "wall" is basically just a way to say "we should enforce the quotas that our immigration experts are setting so that our labor force isn't glutted by excess supply in any relative tier".
This is the republican tack to tackle wealth disparity.
The arguments against this are mostly humanitarian, from what I gather. A significant portion of the population consider's it America's duty and legacy to be the harbor for the indigent. There's also arguments for cultural enrichment etc.
I think that's why so much emotion is involved in opposing it, and why so much backlash is rightfully indignant and angry. One side is hurt by this betrayal of the American identity by building a wall to oppress the poor and keep them from the upward path -- and then begins to call the other side hateful things... When the other side tried to start an economic conversation.
This is just my interpretation of the situation.