The key word there is "estates"; "legal estates" would be more accurate.
Gates et al move their accumulated wealth out of their legal estates into tax-free foundations and the like. So, while their legal estates may be taxed, there is almost nothing there to tax.
No, they didn't sell their stock to fund these foundations - they donated stock, which is either tax free or provides a tax benefit. Yes, those foundations serve many of the purposes of "estates". (The Gates kids and their kids and so on will always have an income.)
There are other things that they can do to shift money to their kids and grand-kids without significant tax consequences. (Yes, there's a $10k limit on some transactions, but ....)
There are things that these folk can do while they're alive to generate tax benefits that they can then use to make other sales tax free, giving them something to live on. (I don't know the legal status of the Google planes, but the planes that are available to NASA could easily generate tax benefits.)