Those unions aren't specific to one company/team though and are mainly to set pay floors because they work in a glamorous industry that people are desperate to

Those unions aren't specific to one company/team though and are mainly to set pay floors because they work in a glamorous industry that people are desperate to get into and so they are used to raise the people outside of the competitive spotlight pay in the industry. If you want to start an non-company specific union for developers, by all means, start one but I see no reason to join it considering the only reason people are desperate to be developers is because it pays well.