The difference is that people associate a union with forced membership; people who wanted to work at Google and to negotiate directly with Google, rather than accepting what the union negotiated for them, wouldn't be allowed to. If the union membership was entirely voluntary I imagine most people wouldn't object.
The other aspect (and I'm not trying to make a strawman here), is I'm getting the "temporarily embarrassed millionaire" vibe from your post. People would object to a collective under the pretext that they are special among the 120k googlers and would somehow be able to negotiate a higher comp than what a hypothetical collective agreement would force on them.
What I found downright comical is this objection comes before the union is formed, before any details about how compensation would be handled is even discussed. So again, it feels like the very people who would be empowered by this move (since it is them who the collective would represent), object to the concept before even discussing the details. All under this uninformed notion that they'll be prevented from partaking in outsized compensation in the future when they inevitably rise to the top echelons of Google.
I call this uninformed, because unless any of these objectors have information, they can't know what the comps would be, since it was not discussed to the best of my knowledge. Nevermind the fact that by definition, most Googlers will not rise to the very top echelons because space there is naturally limited.
This mindset in the US that workforce empowerment is bad has to stop. It feels like the middle class in the US is fighting ferociously alongside the mega-corporations in obliterating the middle class. Corporations are not your friends. The C-suite at corporations, and the shareholders are not your friends. They are not enemies, but because they are more like an amoral hivemind than a single benevolent entity, they'll naturally gravitate towards maximizing their payoff, even if this is at the expense of the workforce. Again, I'm not saying there is outright malice there, it's just the natural optimum state for the a group of entities who currently hold most of the power.
The US is basically a feudal society in everything but the name. If the Google employees manage to get traction and their efforts spread to the other parts of the industry, and perhaps even other industries, and the balance of power tips even just slightly back towards equality, that's already a win in my book.