Part of it is the cost, part is GM has not had any strong 5- or 6-speed autos. They do now, and will be using the 6-speed in a lot of upcoming trucks. I should add that GM powertrain was one of the first to develop a 5-speed auto, almost ten years ago, but GM did not use it; BMW did. For those with a BMW 5-speed, you most likely have a GM powertrain trans, manufactured in France.
BMW did the testing for GM on that one.
As for the 4-speed. The 4L and 4T autos have been around for a long long time, and are usually iron clad. Usually, if they have a problem, it is due to a supplier quality issue.