As well as the fact that taxing unrealized gains would have to come with _issuing tax rebates_ for unrealized losses - something the government would find difficult to do in the case of a systemic economic downturn a-la 2008. And then there's the fact that you aren't really creating any new revenue by taxing unrealized gains if they are going to be, at some point, realized. Last I checked shares of stock aren't very useful as a payment instrument in the general case, nor would IRS accept them as taxes.

This is a cynical ploy to the part of the populace that can't tell a portfolio from a hole in the ground, and there seem to be surprisingly many of such people.

Also, to address GP's comment - Apple would then buy back all outstanding stock and destroy it, dramatically driving up the share price for folks who haven't sold.