It's mainly leveraging cartel power rather than proof of work. Even if you have a lot of experience acting in non-union productions, you're still ineligible from joining SAG. To be clear, I don't think this is a bad thing for SAG because the number of aspiring actors far exceeds demand, so SAG increase wages without causing the most of negative effects with cartels, but I don't think this is the same for tech.
The issue with the "proof of work" standard is it sets up a difficult conundrum. You need to work to join the guild, and you need to be a guild member to get most work.
But the number of times I've heard directors and such tell stories about how they wrote a line for a non-speaking part as a favor to the actor in the role, solely to allow them to get their SAG card, is disturbing.
It's indicative of a high degree of gatekeeping.
It’s great when you’re in but if you don’t have the connections to get in you can find yourself on the outs for years.
If a SWE guild can find a way to eliminate the nepotism and create a meritocratic way to get into a guild…
Not to say working has no value, but neither does schooling have no value.