Earlier quoted context omitted.
Of course unrealized gains won't be taxable, but they can't be spent either. For me, it makes a lot more sense to tax realized profit, or better yet consumption, than wealth itself. I don't think it is desirable to force realization just to increase tax revenue.
My understanding is that you can play tricks with stocks so that you don't really need to realize the gains to benefit from them. They can be used as collateral for loans, directly donated to charities (lowers down your tax bill w/o forcing you to realize the gain), there's also the step-up basis loophole (or was it already closed?). I may be missing some details (and other creative ways to do "tax optimization") tho…
It is true that buybacks are more tax efficient than dividends, which is why companies generally do buybacks instead of dividends.
The US tax on capital gains is definitely a mess though.