First of all, ALPA is not a true union, it's an association. Each individual company MEC is a union, but heavily governed/regulated by the RLA.

Pilot compensation is driven by supply and demand. It takes quite a bit of time/money/investment to make a pilot and training to keep them up to date and productive. That's why even non-union pilots eventually either get close to union wages or adopt a union in order to implement a CBA. If it were "just the union" then you'd see upstarts able to hire pilots at 1/2 the compensation and pay them at that rate indefinitely, but alas, they'd get what's left over after the others had hired. Eventually that strategy would actually lead to higher inefficiencies that would negate any labor advantage you might have at cut rate wages.