Why can't the pleasure of learning be the significant return?
Of course it can. But there are many ways to learn, some more costly than others, so if your objective is to maximize your learning, you may well decide college is not the right forum for learning certain things, while it is for others. For example:
1. A lot of learning is both pleasurable and economically rewarding. For example, much of engineering, mathematics, and the sciences offer economic returns as well as intellectual ones.
2. For pleasurable learning that is not economically rewarding, there are often less expensive ways than college to obtain it. For example, you don't need to pay thousands of dollars just to read the classics. They're readily available at the bookstore for far less. So you have to ask yourself, how much is the classroom experience worth to you, for a specific type of learning? Is it worth tens of thousands of dollars a year more? Maybe - if you want to teach literature at university someday, or have a huge passion for the subject. Maybe not, if you're just learning for pleasure's sake.
3. What different learning could that same amount of money buy you? Is it worth thousands of dollars to take a class about a foreign country, when for the same sum you could actually visit that country? Maybe, maybe not, depending on your goals and opportunity costs.
The real issue with the "become better rounded" argument for college is not with being rounded, per se. A liberal education is great. It's that the argument is often made by folks who do not know where they are trying to go in life, make an enormous life decision without putting serious thought into it, and then graduate and complain how unfair life is that they have six-figure debts and no career prospects. These people are known as "fools", but pretend to be "intellectuals" because admitting foolishness is a blow to one's ego.
On the other hand, if you make a college decision with your eyes wide open, and are willing to trade off your future material well-being for the intellectual rewards of, say, an art history degree, I salute you. That's not foolish, that's having values and living up to them.