Because there is nothing wrong with GM's 4-speed. There are some instances where a 6-speed is really needed but the Camry/Impala segment isn't one of them.
In what list? Hummer being #34 makes me think this is a Consumer reports list. You know that one of the major "issues" with the Hummer is gas mileage right? Yes Virginia, people report gas mileage on their Hummer as an "issue". I know that Cadillac is pulled down because of the 3.2 litre CTS engine that they don't even sell anymore.
That's like saying "The Prius and Corolla are clean environmentally, but the Land Cruiser and 4Runner are the worst polluters in their category".... oh.. wait... woops, that one is true.
Solstice aside because there is only one other non-GM in it's category. We know that Uplander is a dead man walking and I'm leary of the Cobalt's numbers from CR anyway. There are still vehicles like the Impala, Aura, G6, LaCrosse, Lucerne <no 4-speed OR 6-speed failures>, GMT-900s <no snapping camshafts>. Lambdas <no steering wheels falling off>, etc etc etc.
I'm not saying there aren't good reasons to buy a Camry. I'm saying that the Camry's issues in the past <sludging 4-cylinders, failing 6-speeds> don't get published, while Impalas "issue" of a fully functional, smooth, reliable 4-speed gets harped on constantly.
bullox! BMW's 3-series had major mechanical failures during C/D's test drive comparison and they STILL said it was the best.
Reviewer's snide, anti-domestic, comments are well documented in this forum.