Investing in startups returns a couple points over investing in boring things like corporate debt and startups have higher risk. Unless you think more social good comes from investment in corporate debt than startups, it would be foolish to incentivize everyone to ignore startups because the risk adjusted returns are better elsewhere. I accept that some people think investing in startups is a waste of money but I would prefer that our tax code does not force that to be the case.
The argument about inflation adjusted returns is really the baseline case. Most investors account for it. However, they also adjust for risk as well. I would prefer investments in tech startups to be attractive, net, after accounting for the tax code relative to boring investments like debt instruments.
Another factor is that long-term capital gains taxes are much less efficient than income taxes in terms of adverse impact on the economy. If you need to raise additional revenue, it is better for economic growth to take it from income (or sales) than capital gains.