I don't think there's any other reason.
I am surprised they are not just as familiar with PDFs, though.

In any case, recruiters are not techies. To many, there's a program to "type" and the idea to use another one to generate a "better" version of the document they produced is just silly and complicated.
And they're right.
From the user experience perspective, it is just silly that word processor X (MS Word, or whatever other you may like) does not produce a finalized, universally accesible, unchangeable version of their work.

So, one may argue that Word does that, but it is not obvious nor user friendly.
One may argue on what is universally accessible, and unchangeable. But the point, from the user experience, remains.

I share completely your frustration and your arguments. I happened to interview people in the past, and never wanted a CV on Word (which I like, and use daily); I always asked for a PDF version, for exactly the reasons you mentioned.

I was amazed to how many people produce WordPerfect, or MS Wordpad. Once I got a CV in HTML, once a PPT presentation file, and once I got one in Flash.

But, I think you'd be wrong to expect that somebody who uses a PC to type letters and sees it as an advanced typewriter which occasionally can be used to listen to music and browse the net, may ask for any other format than the one they use daily to produce text (<- not it jargon) documents.

My advice: get over it: while technically non-optimal, it is perfectly normal that people want Word; just don't be irritated, see if you can convince them to get a PDF - which seems to me perfectly reasonable -, and if you don't succeed, just produce a Word file. I'm sure there's converters out there for those who don't want to install Office on their desktop computers.