I think it's because technology is incredibly powerful and code + hardware can act as not only organizational structure but as a sort of pseudo employee. When you code you are often designing and implementing business rules, thus acting in a managerial or executive capacity. Software also scales incredibly well. One person may be responsible for building a system that generates immense value (extreme example: Linux and Git both initially created by one person).

The level of responsibility and authority you have as the person responsible for a $10M system can be leveraged to move to different leadership career tracks. I think this bleeds out into the industry at large so VCs recognize that smart engineers make good financial bets.

My hypothesis is that this is a function of white collar office work, where one often works closely with management/leadership/mentors, and can build relationships, knowledge and skills that increase one's opportunities the further one progresses in their career, not only in terms of entering management, but also starting a small business or startup.

Unions historically stem from segregation between labor and bosses along multiple dimensions such as skills, day-to-day experience and class.

Because it's a professional job despite not needing a formal degree, a and professionals can start their own own businesses.