Because you don't ever sign an agreement with them. They just collect your data from all the other entities you do business with. Your bank, your landlord, your employer, your credit card company, etc.

Not that they haven’t tried! Equifax snuck an arbitration agreement in the free credit monitoring they offered in the wake of their hack. They removed it after (understandable) uproar.

That would require you consenting to their spying.

It's possible to consent by proxy.

For example, if your employer uses ADP for payroll processing, you consent to ADP selling your wage and income history simply by working for that company.

I can't imagine a job interview that would include, "Oh, by the way, if you take this job, we're going to let a multi-national megacorp know every penny you make, every two weeks, and sell that information for tracking, profiling, and advertising. Also, there's free coffee next to the men's room."

Seems likely that nobody at your employer realizes this either.