I've heard of this argument. 'Why do the rich need so much money? Why not just create a 'wealth cap'? The problem is, capitalism is about incentives. When you create a negative incentives, it creates externalities, possibly in the form of companies moving overseas, job loss, and economic stagnation. Second, is it fair for someone or some entity to confiscate what is not theirs, beyond taxes? Third, capitalism is expensive. Venture capital is funded by wealthy, an example being the Space-x and Blue Origin rocket programs, both very costly and funded by billionaires Musk and Jeff Bezos. If taxes were much higher, such programs may not exist.

Most venture capital is private:

http://si.wsj.net/public/resources/images/MK-CQ161_KEYWOR_16...

http://www.wsj.com/articles/humanitys-last-great-hope-ventur...

High corporate tax rates create an economic incentive for companies to move factories and offices overseas through 'inversions' to get a lower tax rate:

http://www.economist.com/blogs/economist-explains/2015/08/ec...

Relative to most developed countries, America has a very high corporate tax rate:

It’s a rock-solid fact that the U.S. corporate statutory tax rate is the highest among developed nations and is significantly higher than the average. According to 2014 data from the OECD, the combined federal and state statutory corporate tax rate for the United States is 39.1 percent. The average of the other 33 members of the OECD is 24.8 percent — 14.3 percentage points lower than the U.S. rate. Weighted by country GDP, the average for these 33 countries is 28.3 percent — 10.8 percentage points lower than the U.S. rate.