1. Tech workers are currently treated as well as (or better than) we would be without unions.
2. Unions would cause companies to offshore jobs to developing markets with similar talent.
If unions don't increase worker compensation, why would they cause companies to offshore jobs? Conversely, if companies could acquire comparable talent in emerging markets for less money, why aren't they doing that already?
† Or, rather, they could be, but it would mean companies leaving a lot of money on the table out of the goodness of their hearts.