The disingenuous aspect is simply having a canned response intended to induce positive feelings (intended to make them like you?) regardless of whatever they are saying. Even if it's often factually correct (e.g. "Everyone thinks they do difficult work") I still think it's insincere to have this line up your sleeve that you march out regardless of input from the other person.
"What do you do for a living?" is a canned question to begin with. It's what you ask when you don't know what else to say. People in social situations usually aren't there to talk about work, they're often there to get away from it. The point of the canned response "oh that sounds hard" is to give the other person an opening to talk if they want to, because the phrase signals that they have a sympathetic listener, but it doesn't require them to do so (because you didn't ask a direct question). It also signals that you aren't about to start asking direct questions about their job, which is something they probably don't want (because, again, this is a social situation).
You may decry it as "canned" all you want, but it's a phrase designed purely to allow the conversation to flow and to make the listener feel like they aren't being put on the spot. I bet you have a bunch of canned questions/phrases you trot out in social situations too, for various reasons, you just haven't catalogued them.
