A large portion of their wealth will never be sold off, Bezos isn't going to sell most of his Amazon stock and therefore would never get taxed on it until he dies and it becomes subject to an inheritance tax. Since he could easily live another 30-40 years, should the government really have to wait that long before collecting taxes? And even then what about a guy like Gates who plans on donating all of his wealth, in his case the government won't ever get their slice.
As to whether it's easier or not, it's questionable, either way there will be major opposition so you might as well go for whichever tax is better on the merits.
Thank you for pointing out that we already have a wealth tax called the inheritance tax! Should the government have to wait? It's getting the money either way and people die each year so it should even out. Consider, if you kept taking 1% off the top that slows down compounding, which means he wouldn't have as much to tax when he passes. Mathematically it works out similar to a mutual fund with 1% expenses and we have all seen the math on how that can cripple long term growth. I might have to math that out, how much would a wealth tax decrease the inheritance tax revenue once you consider compounding?
I think you also identified a likely unintended consequence of this. Super philanthropists who plan on gifting away 90%+ of their wealth, like Gates and Buffet, might speed up their gifting to put even more money into their foundations to limit the wealth taxes.
Math:
1. Estate tax
PV: 1,000,000
Growth%: 5% (being modest)
Time: 30 years
FV: $4,321,942.38
Estate tax: 40% $1,728,776.95
2. Wealth tax 1%
Had to use a spreadsheet to take into account the payments keep changing since they are 1% of the new value each year. Still assuming starting value of 1 million, 5% growth, 30 years.
Total tax payments at 1% of value each year: $560,849.38
Value at end: $3,243,397*
Estate tax: 40% = $1,297,359
Total of estate tax + wealth tax: $1,858,208.
Okay, the wealth tax + estate tax does give you higher total revenues, BUT you went through all that political trouble to increase the total cumulative revenues over 30 years by 7.5%? That's a lot less than 30%, which is around what you would expect out of a 1% wealth tax X 30 years. Adding the inheritance tax(our existing wealth tax) back into the total calculation shows an additional wealth tax is likely not the best way to raise revenues.
* I used a spreadsheet to make the annual payment calculation easier, but I also used a TMV equation to check my work at the end. I still got $3,243,397 as my future value in case anyone wants an easy way to check the math.
PV: 1,000,000
Growth%: 4% (5%-1% tax)
Time: 30 years
FV: $3,243,297