Even with RSUs, you'd have a tax obligation on illiquid RSUs if not for the terminology used in the contract to delay the actual "vesting event" in order to delay a tax event.
Depends on where you are - I believe that's the case in the US, but for example in Australia when RSUs fully vest the value at vesting is included as income on your tax return at the end of that financial year, so you don't actually pay the tax until after you file and are assessed.
Right, they're withheld due to adding a clause to achieve this.
Without that workaround, RSUs would intrinsically trigger a tax event.