I suspect a fair few of the people who do this are trying to trick search engines into thinking their content was posted more recently than it actually was, sin

I suspect a fair few of the people who do this are trying to trick search engines into thinking their content was posted more recently than it actually was, since a few SEOs did experiments and found content without timestamps/with misleading ones would sometimes rank higher than those with accurate ones. That or they think it gets people thinking they post more regularly than they actually do. But yeah, agreed. Especially given that software development is a rather time sensitive field, and what works/happens to be best practice at one point may not be so further down the line.