"They've already got all my data!"
Maybe, up till now. But...
As impressive as big tech's profiling models are, they still need to be constantly updated with new fresh personal data. If you stem the flow of personal data, you starve their models, and the whole machine is not so powerful.
That creepy digital model of you that they've built? It can fade over time, lose its grip on you. When your uncanny avatar is not up to date, its value to big tech is less. Their grip on the advertising market is weakened (advertisers will not be able to micro-target so accurately, and so will pay less).
The surveillance business model relies on collective apathy
And the opposite is true: it falters with collective action. If we are all careful to starve the system of our personal data then you hit them in the wallet. If just one person acts, there is no effect on big tech, but if millions do? They need us - we provide their raw material for their machines - so we have power en masse.