I'm not sure ABET is the most relevant here? What would making CS programs accredited change?
And you didn't quite mix it up, a degree from an ABET accredited school is required to get your professional engineering certification, which is often required in cases where engineering designs carry legal liability.
I am unable to find concrete numbers on this, but I would bet that the majority of employed engineers in the US do not carry a professional engineering certification.
My point is that accreditation boards and trade unions can help set standard minimums of what an engineer (or trades person) will know going into a job. Of course employers can ignore them, but in general those standards help bring consistent expectations to a given trade.