They're talking about solving problems on a scale of a country the size of the US. A few extra bucks from some small number of wealthy people is meaningless.
That implies that the tens of billions of dollars spent by the rich on charitable donations wouldn't make an impact if given to the government, doesn't it? If that's the case, why tax them for that amount?
The 2019 federal budget was $4.407 trillion. let's say we're talking about $20B in voluntary tax overpayment (good luck with that!), which would represent .004914% of the budget. That would pay about 3% of the interest on US debt in the last year. It would change nothing.
However, increase the marginal tax rate and you're talking about real money. Maybe in the meantime we can spend a little less on the military, but that's a tough sell too.