Piketty and his collaborators being a notable and significant exception. https://eml.berkeley.edu/~saez/piketty-saezNBER12optKtax.pdf

Taxing capital is hard, because it's mobile. Governments are afraid that if they tax wealth at a high rate, the wealth will just move elsewhere. Thus, most businesses get some sort of property tax abatement as part of an incentive to locate in a particular city or state.

But taxing capital would still be easier than taxing robot work, yes? Since taxing robots is effectively a tax on a difficult to define subset of capital

Not arguing with you there -- taxing robot work sounds almost nonsensical. If anything it would be a great jobs program for tax lawyers who will tie themselves in knots figuring out how to circumvent this ill-defined tax.

Taxing capital is very straightforward from a regulations standpoint. It's only hard in that the capital will tend to flee the tax.

Mooting a tax on one of the more futuristic by-products of technology makes you question both the nature of taxation and the essence of technology.