A few factors that go it to it are that it is a relatively low barrier to entry to get in to, so you have a lot of people that aren't very good employees. So weeding through the bad ones can be expensive because they can be flaky, get in accidents, etc.

Also, about 3 years ago now, there were changes made to how drivers run their log books, as they only have a certain amount of time to drive daily. Lots of regulation there. With the change it mandated a change to electronic logs instead of paper logs for the vast majority of over the road drivers. It's great in a lot of ways, but there are some aspects that really can cut down on the amount of time that a driver can drive daily (most companies pay per mile driven) which decreases their money making ability. Many older drivers basically just quit when that happened because they didn't want to deal with the changes.

Lastly, most insurance companies won't cover a driver with less than 2 years of experience, which means that most new drivers are funneled in to the larger companies (Swift, CR England, etc) who pay maybe 50% of what most smaller companies pay. They also usually make them drive teams, so they are stuck with someone else 24/7 who they may or may not get along with. Similar to a mission except you are stuck in a truck with them and the truck is traveling a ton. So these drivers will be making ~$50k to $60k/year, but they are working 60-70 hours per week, and they are on the road for weeks and sometimes months at a time without much of a break. I was recently listening to a seminar of truck driver recruiting, and they said that their data is showing that for every 4 drivers that get their CDL and start driving, only 1 makes it to 2 years of experience where they will have the opportunity to leave those big carriers and go to a company like ours that pays a lot more and they can have a lot more home time.

The problem is that the amount of freight is going up, and the amount of drivers is going down. I can't blame the drivers as unless you like that lifestyle it's a pretty crappy job for the first couple years, and even afterwards can be rough. Anyways, that just my $.02 on it having been recruiting drivers for about a year now.