I think (and this is not based in reality at all, I'm just guessing) that a lot of the jobs mentioned - like mail room - either no longer exist, or have been outsourced to 3rd party providers where there simply is no upwards mobility beyond supervisor, but only at that 3rd party provider. I'm thinking things like administration, call centers, accountants, etc.

A lot of work has been compartmentalized, and it's created additional barriers.