One thing I don't see discussed in "tax the rich" discussions about US politics is that it actually often means "tax the capital". But capital is very mobile in modern world; the moment you increase tax on capital gains in US, markets rebalance, and capital moves to other parts of the worlds, reaching some kind of risk/gain equilibrium.

For me, an outsider, the US was always in contrast with other first world countries in terms of how lucrative it to the capital, and how powerful and healthy the whole economy is on a large scale, and how quickly it recovered after 2007, compared to other economies that were similarly affected. Everybody moves to US for jobs, or to start a business. Doesn't US left wing sees that as something worth preserving?