$300k of investment yields roughly $1k a month, which is doable in the US. You just have a studio apartment in Indiana or something, and do very little other than cook very economically.
You do, but the pile of investments is either generating income or losing value to inflation, so in practice it's not that much of an issue. You can't spend tax-deferred retirement accounts like a 401k or tIRA without realizing taxable income, and there's significant contribution limits for Roth IRA balances for tax-exempt gains. You can defer capital gains but not dividends, so a large stock portfolio doesn't help you much either.
Plus if you manage your income to stay under ACA subsidy limits, you're generally going to be paying a higher effective tax rate by forgoing tax-deferral opportunities at higher tax brackets in your income earning years or Roth IRA conversions to use up lower tax brackets in your post-retirement years.