Because consent must be "freely given". As soon as you start attaching consequences unrelated to the utility itself, you're making a decision less and less freely.
The greater the power imbalance, the less free the choice. Social networks are a great example of this. You can choose not to use a particular one, but what's the alternative if everyone is already on that platform? You can go without, but what if it's LinkedIn, and there can be a real impact on your career?
The rest of what you wrote is silly. Social media websites are not charities. They don't have to provide you with a service if you are not willing to compensate them with your data.
Personal data is not the only form of compensation, and GDPR is a direct response to the situation that attitude has created.
Nobody is suggesting companies provide free services. We're saying that personal data is more than commodity, and we should be looking to more ethical business models. And we won't be sad to lose companies that can't adapt.
edit: And I don't think my point was silly, but I'm also not really libertarian. So I don't think it's acceptable for companies to abuse their dominant position to make things worse for society at large.
You're making a philosophical argument about what is a "real choice", precisely the problem with the "based-on-principle" GDPR. All this will do is create a big mess if/when this gets into real litigation.
The greater the power imbalance, the less free the choice. Social networks are a great example of this. You can choose not to use a particular one, but what's the alternative if everyone is already on that platform? You can go without, but what if it's LinkedIn, and there can be a real impact on your career?