That's not true where I live, "used" extended cab pu's are all over the place! Thing is, they now have "shortened" forward facing doors instead of suicide doors with no post.
The reason GM added the post, is to get a MUCH higher rating in a roll over crash, as that post supports the roof MUCH better.
You see them as a crew cab, but that's an even longer animal.
SR
I'd like to point out that it was Dodge, not GM, that invented the "extended cab" with the smaller rear doors and a full B pillar. GM didn't begin using the smaller but normal rear doors until 2014; Dodge was doing it at least as far back as 2008.
Furthermore, if you compare a 2016 GM 1500 "double cab" (with the normal rear doors) to a 2016 Ford F-150 extended cab (which still has the clamshell rear-hinged doors), both are rated as "Good" for roof crush integrity.