Tax cuts do result in more economic activity. We've seen that. We also know from watching human behavior that when you tax something, like economic activity, you get less of it, for the simple reason that you are increasing its cost. Remember the yacht tax? We were really gonna get dose rich dudes, yeah, by slapping a 10% tax on those expensive yachts they buy. Homework exercise, how much new revenue did the tax generate and did the wealthy buy fewer yachts?
The point is, humans alter their behavior to avoid paying higher taxes and tax increases never generate as much new revenue as they're projected to because those who write them assume people won't change anything and will keep doing what they've been doing, only paying more. Want to jack up the capital gains tax? You'll get less investment. Jack up the inheritance tax? You'll see people dying with their net worth just under whatever level you set, but their heirs somehow will still have the wealth. Jack up a corporation's taxes? Prices will go up. It's what people do.