> then you might be considered an employee anyway. Which I can't understand why you _wouldn't_ want that
Because it comes with a significant reduction in flexibility. If the employer has to provide health insurance then they'll pay you proportionally less, even if you already have health insurance through your spouse. If they have to pay you overtime then they may cut your hours instead, or lower your base pay to compensate, which may be worse for you than being able to continue working at the original wage.
Meanwhile if you did prefer some aspect of the arrangement the law would require then there is nothing stopping you from negotiating that to begin with in exchange for the correspondingly smaller amount of cash money. Few employers have any objection to providing you with health insurance in lieu of the same amount in cash that they would otherwise still have to pay you in order to get you to work for them.
But some employees would prefer the cash -- and the employers take some of the loss when not being able to do what both parties prefer makes the employee walk away, so they don't like it either.