Because complexity isn't good. We already have an insanely complex tax system.

In my opinion, we don't need perfect and we aren't comparing to good.

A perfect tax code would be impossible, a more simply one would be very doable.

We're talking about a campaign proposal here with no legislative draft so its a guessing game, but in my opinion any move similar to taxing unrealized gains will serve only to make it more complex and would not fall under the category of "good" for me.