We (Michigan) used to have to retake the written portion of the test every 4 years (when the licenses expire), now that's not even required anymore.
The better question...who would pay for such a test. The DMV/Secretary of State can't hire enough people to handle the lines in the offices let alone send some of them out on the road for a test.
Last time I went to the local SoS office, I was 10th in line, and there were 3 people working in the office at the time. A decent ratio (IMO).
However, at any given time, one was outside smoking, one was on the phone, and one was actually helping customers.... and they would rotate...with there never being more than one on the desk... I wrote a letter to SoS that day...never heard anything back though.