What laws, don't you have freedom of association in USA? Just create a group, what is stopping you?
Speaking as an ignorant European.
For more of the legal perspective, see page 9 of: https://docs.house.gov/meetings/JU/JU05/20191029/110152/HHRG...
I don't believe that at all, mandatory unions are illegal in Europe yet unions are stronger here. It wouldn't surprise me if making them mandatory actually makes them weaker since workers are so afraid of creating unions that could potentially harm them, while I can always ignore an optional union so they aren't a problem.
I don't think requiring unions is the only way to do it, but the countries that have a high rate of unionization seem to have other incentives in place, such as Sweden and Denmark where unemployment insurance is generally provided through unions in contrast with Norway or France, where this is not the case. The latter countries perhaps consequently have much lower rates of membership, closer to that of the US.
https://www.vox.com/policy-and-politics/2017/4/17/15290674/u...
Traditionally the answer to that is unions or professional group who can bargain on behalf of all their members, but it's hard to unionize fields with low barriers to entry (and harder when the workers are living paycheck to paycheck and thus can't strike easily), and impossible to unionize independent contractor drivers based on how the laws are written.