How is an engine mechanic more 'blue collar' than a sysadmin or a website 'mechanic'? And a lot of coding could be (and has been) compared to assembly-line work.

I think the only difference, in this respect, is that one technology has been around longer and the other is still the cool new thing. Someday, in a forum about whatever the new thing of that moment is, someone will ask the same thing about people who work in IT.