Why so many cdl driver openings?

Because it's an extremely high turnover industry. These major freight carriers have effectively turned these trucks into wheeled sweatshops, and people either find out the hard way that they can't hack it, or they get tired of it and move on to other things.

The "driver shortage" which the ATA is citing is largely a manufactured thing. Megacarriers offer in-house schooling - just enough to get a license in your hand - and they get paid by the government for it. Then, on top of that, they either charge the students or require them to do what basically amounts to a period of indentured servitude to pay the school costs. If they actually trained good drivers and could actually retain people, they'd have no need for their schools.