No, there is just no legal recognition of a union (as is understood in the US) for freelance contractors. Contractors organize through professional associations, instead.
I'm sure it's a lot more complicated than this... but when a majority of employees vote to unionize, and the union is certified, the union then has sole authority to negotiate employment terms with the company. I think from that perspective, it makes sense why independent contractors, who are by definition negotiating directly with the company, can't unionized.