That the wealth of a more educated society compounds, likely in super-additive fashion. Basically, the idea is that a society full of educated people is wealthier as a whole and thus in your own interest. In the short term, it is easier to be well off if people around you are well off than if not (e.g. is way easier to be an entrepreneur or to find a well paying job if you are surrounded by other educated people). In the long term, your society gets richer and, since countries that have public education also have things like public healthcare and social security, a richer country is also very directly in your benefit when you are nearing retirement.
Note that the U.S. already buys this argument when it comes to primary and secondary education. It only seems to mostly ignore it when it comes to the university or trade school level.
The U.S. gets by without public higher education, but that might be in significant part for two reasons: accumulated private wealth in a large middle class that got rich from globalization with one of the few intact industrial bases in the post-war environment (now dwindling), immigration of educated workers from all over the world (brain drain from other countries).
Also, note that many countries with public higher education see it as a system where each individual "pays it forward" for the education of the next generation, at a moment in their lives where they have significant more economic safety than they would in their late teens. Simply put: "I'll pay for the education of the next generation, because the previous generation paid for mine". This is something which the U.S. does within families anyways, but some other countries do collectively, resulting, again, in compounded gains from a more educated society and cheaper education costs due to collective bargaining. Things like fancy dorms and millionaire school administrators are hard to justify when they are paid out of the paystub of your average nurse and construction worker...
Don't get me wrong, I did my undergrad in a (developing) country with public higher education and my graduate studies in the U.S. (on school support). There are definitely benefits to the U.S. system if you have the money or are in the top percentile for talent/luck. But overall, for the majority, the lack of a strong free public education option is a huge disadvantage, it seems to me, even after six years here, that this is one of those things where the U.S. does well despite of it, rather than because of it. Like private healthcare and lack of public transportation.