It's supply and demand. Kids don't grow up wanting to be CRUD app developers, they grow up wanting to be game developers. This means that working conditions are

It's supply and demand. Kids don't grow up wanting to be CRUD app developers, they grow up wanting to be game developers. This means that working conditions are notoriously bad (bad pay, terrible hours). To raise pay they need to constrict supply, to constrict supply they need to put up barriers to entry. Actors union is a definite barrier to entry: you need to either be a notable independent actor/performer (rare), get a job on a SAG AFTRA because you have a talent that they can't find in union (rare), or show that you've worked in the field (non-union commercials/films) enough to be notable. Then you need to apply, pay $3k, pay ~$200 a year dues, and give up ~1.5% of all your contracts to the union. Then you can't take a non-union job (again restricting the talent pool). I honestly don't see a ton of difference between the two: both establish choke points and collective bargaining in high demand fields to improve working conditions for the people that are in the union.