Because the US is a for-profit society, not one focused on improving the lives of citizens. Companies are making billions on health insurance, education, etc.
It won't change because they want to hold onto their profits.
In that case why is school free?
Elementary and secondary are free to keep people going to work (i.e. it's child minding), and to keep children out of the workforce.
It's not free, it's subsidized by the government. An important distinction.
Is anything in this economic system "free" ?
School taxes on property pay for K-12, whether you have a child or not. I'm not ready to pay many multiples of that to send everyone to college for "free" for the rest of my life.