College degrees are better signals for conscientiousness than intelligence,

I doubt this is true. I've seen research that claims that on average, college students spend less than an hour a day studying. I've attended 3 universities in my life (undergrad to grad school), and skipping classes frequently, dressing like a slob in class, and skipping the required reading seem typical. If I cared mostly about conscientiousness, I would be more impressed by someone holding down a job at McDonald's for 4 years than graduating college, because a McDonald's manager has no problem with firing someone who skips work frequently. Most college professors don't even take attendance.

Yet long-term McDonald's employees get very little career boost from this in applying for jobs at Goldman Sachs or whatever. A kid who manages an Art History degree at Harvard while mostly partying and doing the minimal work to pass has a vastly better chance than the a long-term McDonald's employee with a sterling letter of recommendation from his boss.

which is no coincidence, since employers in real life care more about conscientiousness.

This is vastly over-simplified. I did an internship at a firm that designs employee-selection systems for businesses, and this varies widely. A company like Walmart or McDonald's doesn't care much about intelligence, they want reliable, polite workers who won't steal from them. On the other hand, intelligence receives a huge premium for high-level white collar work. For these kinds of jobs, beyond some reasonable level of conscientiousness, they no longer care, and more conscientiousness demands no premium.