Even if they sold everything they're limited by regulations to a few percent per year. Since billionaires have maybe a few decades left to live its not even possible.
If they donated shares then the charity would have the same problem and give them back to not be accused of manipulating.
Assuming its in bank account there is a very low ACH limit (few thousand per day), and checks take time to order and have their own limit on what will fit in the amount box.
Is it even possible for billionaires to spend their wealth in their lifetime?
Here's an example that was given: Billionaire Chuck Feeney has finally given away his entire $8 billion fortune after making secret donations for decades.
It took 40 years to give away 8 billion, and its unlikely to be an actual donation because he probably still controls some part of those charities and has enormous connections built on that.