But people don't see that, because it isn't true.
You're conflating the idea of external remote control of software with freedom to do what you want on a computer. They're completely different, unrelated concepts.
Of all the operating systems, only Windows does things that it really shouldn't, and resists attempts to keep it from doing those things.
macOS puts limits on non-expert users, but if you know what you're doing it lets you work around the limits - so far, at least.
Even iOS is easy to open if you know how to install fuse from the command line. You can't run as root, but you can access the file system without having to go through iTunes.
Ubuntu meanwhile added links to Amazon that no one really wanted. In theory you could get rid of them by recompiling or using a fork. But creating and maintaining a custom Linux fork is something that is completely beyond the skill of 99% of users, and probably beyond the skills of more than 10% of professional developers.
And if you're using someone else's fork, you have no idea what's in the code. We've seen libssl infiltrated, and it wasn't even hard. Who knows what else is in there?
In reality access to source code gives you absolutely no protection from bad actors. There are no reliable tests for overall security, and the ecosystem isn't large enough, focused enough, or professional enough to appraise security with any confidence.
So the idea that "You can control your computer" is just rhetoric. You can't. You will never be able to. You will always need to rely on an ecosystem of developers, and large parts of that ecosystem will be opaque and closed to everyone for practical reasons.