When someone exists in a society, they implicitly agree to all its laws and regulations of the society, such as the state. Since a state is created by the people, they are in a sense agreeing with what everyone else around them already agrees to.
Taxes are simply a means to fund the mechanisms of government for a society, anything from basic government functions, military, or welfare. For these systems to be paid for, the money must come from somewhere. Since people use the government to help structure their lives, such as using its infrastructure and emergency systems, it only makes sense that they should pay to use them and build them.
Some may even go as far to say that someone merely existing requires them to care for others and no one can truly, at least morally, be separate from everyone else, and that everyone has an inherent “buy in” towards society as a whole. However, I don’t generally hold this view, although I will say that for a society to function well and for people to live their best lives, its likely that people must work together and form bigger groups to accomplish this task, which in turn would create a state that requires tax revenue to function.