That's because Ballmer couldn't sell much while he was CEO (that would look very bad).
Well, even if he needs capital, I am sure he can get debt at much more favorable rate than the tax he would have to pay if he sold the shares. The only argument for selling is if he wants to diversify.
Another example is the Walton family: they still keep >50% ownership of Walmart and live off the dividend.
Wow, didn't think about that!
But why give the IRS and the state 50% of it via taxes? He gets a nice chunk in dividends at very low taxes. And the mega rich borrow against the stock so when they die that debt is taken out from the estate without paying taxes. (Ellison got an earful from his accountant for doing so, lotsa debt and refusing to sell his shares. If the stock crashes, you're royally f-ed as the bank demands new collateral...death spiral etc etc)
And what is he going to do with all that money?