Wealth tax proposals I've seen don't kick in until $50 million or $100 million. This means that there is a floor on how "poor" the government can make you via a wealth tax.
This has two implications:
1. Most "successful" startup founders don't break that threshold of personal wealth.
2. For most startup founders, the startup is the only way to get to $50 million. The practical lifestyle difference between $50 million and $1 million is a lot larger than the difference between $50 million and the unicorn-founder $ billion.
Furthermore, as noted by glutamate: money earns money. A conservative drawdown of 3% pay the most commonly proposed wealth tax while still leaving you wealthier at the end of the year.