Why does Deacon drive a truck for a living when he went to college?

Going to college does not, and never did, guarantee a good white-collar job. Not only that, but plenty of people with no college education have risen to high positions in all types of businesses, and still do. As far as millennials are concerned, those who learn useful skills or get college degrees in useful fields have prospects as good as anyone else has ever had. Unfortunately, too many tend to get degrees in social "sciences" and other soft fields that are of little or no use in the real world. Then they sit around sipping fru-fru coffee and making bitter pronouncements about "Boomers".

Office/Corporate jobs aren't all they are cracked up to be and depending on what type of person you are driving a truck around affords you a lot of freedom and spares you office politics. The pay can be comparable as well given that most office jobs are also lower end pay unless you start to climb the ladder in which case they will want you to put in a lot of non paid overtime which they feel is covered by your salary.

Not everyone who works a low end job is there due to lack of prospects.