Earlier quoted context omitted.
You actually can! If the government doesn't protect your wages, you are left at the mercy and greed of capital. I can guarantee you that means you will be thrown under the bus the first opportunity they see fit, as soon as it makes a financial sense. Look at the avalanche of layoffs that started with Twitter. Now the market is flooded with engineers, which makes our individual bargaining position worse. Employers not…
Is fastmail in your mind a megacorp? Most things you said do not seem to relate to a small company with ~400 people according to linkedin.
What is stopping Fastmail to employ the playbook of the megacorps and do a couple of rounds of layoffs? Or to simply stagnate the wages of their employees, citing "bad economic headwinds"? Or to fire some high earners and replace them with cheaper ones from the huge pool of the qualified jobless? That's right, nothing stops Fastmail, unless its employees are protected either by law, regulation, or belonging to a union.
In Australia labor protections, while not as woefully inadequate as in the US, are still not enough to give workers a good guarantee of safety and bargaining power. Hence, they formed a union. It's as simple as that.