A large part of the United States' economic leadership is specifically concentrated in the tech startup sector.
Whether or not you think any of the companies funded by YCombinator[0] are actually worth their valuation, you have to realize that there will be fewer such startups if a tax on unrealized capital gains is passed, and that VC activity, along with the future startups chasing their money, absolutely will move to countries without such a tax.
Again, maybe you actually believe the startup scene in the US is worthless, in which case, go ahead and advocate for an unrealized gains tax Just be honest with yourself that it will entirely shut down sectors that others view as critical to the country's future dominance.