It is much more difficult to enforce the legal obligations of self-employed people than it is to enforce the legal obligations of employees.
Self-employed people are much more likely to cheat on their taxes, are much harder to involuntarily collect debts from, and can look the other way with impunity (or at any rate a low risk of being caught) at all sorts of legal regulations of business. Perhaps in recognition of this fact, the self-employed are expressly exempted from many business regulations.
How much is the size of the self-employed population impacted by the basic unlawfulness of this kind of work, and how much do other factors come into play instead? How would one even go about measuring something like that in a way more sophisticated and immune to reporting biases than a survey?