I think you vastly underestimate the costs of real businesses. Looking at BizBen.com, maybe 3% of car washes for sale are under $100k. The average McDonald's grosses $2.2m/yr and is reported to have profits in the 7% range, which would be ~$150k/yr. There's no way you can buy $150k/year in income for $100k.
You also don't account for the opportunity cost of the time and energy, or the increased risk. If I'm taking my investment money and buying myself a job, then if my investment fails my job is gone too. For a lot of people it's better just to put the money in a (tax-advantaged) 401k and spend their energy on what they're actually good at, which is probably not the evaluation and operation of small businesses.