Do they actually walk out with more, though? Looking at the 'businesses for sale' listings here in the UK, I can see most of the hairdressing salons have an asking price of between 2x and 10x the statutory minimum wage. So even if the hairdresser built the business by themselves in only a few years, has never sold any stock, and has no outstanding loans, they'd probably get no more than twice what they'd have earnt as a non-entrepreneurial hairdresser over the same period. Then that will also be taxed at a higher rate.

I'm not saying that being an entrepreneur isn't a good way of making money in certain circumstances, but I think that starting any brick-and-mortar business is barely viable compared to persuing a white-collar career where one's final salary would dwarf the value of even the most successful hairdressing enterprise.

TLDR; the promise of money alone cannot motivate someone to become an entrepreneur.

This is different to OP's question. OP is specifically asking why it can't be so that the entrepreneurial hairdresser is guaranteed, in some moral sense at least, to get no more than once what they'd have earned as a non-entrepreneurial hairdresser.