It's still a successful industry-wide union. Actors can work a job, finish it, get employed by a different project, and still be represented by the SAG.

Firstly that's just a good trait to have. It's also one of the traits that plays really well with tech workers in general (point 4 above). And importantly, many tech workers are contractors or freelancers, or rely on hopping between companies for career advancement, so that's an important feature to them.