Neat thing about this type of organizing is that the union provides training and standardization paths. Both of those make moving between jobs easier.

They can also provide a standardized way of differentiating employee levels (e.g. union sets the standards for what a jr, sr, or whatever is). I'm not sure if that is good or not in tech, but it is a possiblility - and it's something that would definitely help employers too: rather than each company having to test each potential employee, a union certified X dev will have a certain skillset. Yes some X devs will be better than others, we're talking about humans here, but the minimum bars can be defined and the whole hiring process can become easier and more efficient.

Theres some interesting compensation challenges to get the idea of unionization more accepted in tech tho: stock based compensation and bonuses can get really tricky - something that I suspect is the real reason unions don't catch on more in tech.