- Loosely speaking K-12 can be seen as a public good because of which it may make sense for the society to pay for other people's education. College education is not a public good in the same way because at any rate only few folks will go to college and it is immoral for the other people being forced to pay for college education of those kids. If college educated kids earn even more then it is even more immoral for poor people to pay for rich.
- Unlike K-12, college education involves specialization. A gender studies degree is worthless compared to say a nurse. But because education is free a lot of students might enroll in more and more worthless degrees. This will have great negative impact on productivity of US society. A lot of folks who do not have any productive skills, a lot of folks staying out of labour market in their crucial years. I will work at a local farm for a year rather than pursue some of the college degrees any days.
- When government pays for education it distorts the market. You can see it as a subsidy. But then it also means more and more worthless colleges which have more and more worthless degree programs that focus on "good life" for kids.
I always watch this video from time to time : https://www.youtube.com/watch?v=w3-_r_t7AZU