Here's my theory based on what I've seen in the industry:
A decade or so ago, you had an IT department that consisted of a couple programmers, a DBA, a sysadmin, a couple operators, a pc support dude. Management decided to do cutbacks so they axed one of the programmers and both operators. This meant that the one programmer was doing the work of two, and the sysadmin and pc support guy had to also do the operations tasks. A year or so later more layoffs, this time axing the sysadmin because the servers basically run themselves, right? And the pc support guy had to pick up those tasks. Next round of layoffs, they get rid of the DBA, forcing the programmer to also do the DBA work. So now we have this configuration:
one programmer who does the workload of two programmers and the dba work
one guy (formerly pc support dude) who does the pc support, server admin, network admin, operations
That last guy finally burns his bulb and quits, so management thinks the only way to replace him is to advertise a job that consists of all those tasks combined. That's what his job was, after all. This is greatly oversimplified, but maybe you get the point. You can also add in business analysis and project management work because those are often combined with other jobs. I've seen way too many PM job ads that clearly described a lead developer job with PM duties, and too many BA job ads that described a PM with analysis and engineering duties.
It gets hairy when instead of asking for broad skills like I outlined above, they advertise for specific tools and call them skills. This is a different problem, but exacerbates this problem. Demanding experience with specific tools is what turns the above unreasonable requirements into utter ridiculousness - thus, a purple squirrel.