Before I get swarmed by e-mails and comments from angry artists and illustrators, let me first say this: I'm not saying that the work of designers and illustrators is worth nothing. I'm not saying they shouldn't be paid for their work. Read on, please.
Google, like any other company, is for products and services. If they need a designer or an illustrator for a specific project, I'm sure they'll hire one or pay an outsider to do it. There's a reason, however, why they aren't offering monetary compensation for skinning Chrome. Google didn't set the price for such work at (nearly) zero; the community did.
I know how hard it is to create a skin or a theme for an application or a site. I've created themes for Winamp (that was really tough, with Winamp having a lot of tiny graphical elements making it quite complex to skin) and Litestep (which included learning the syntax of step.rc), and these projects can take weeks of very hard work. Yes, I did these for free and for fun, but then again, I'm not a professional illustrator or a designer.
However, even professional illustrators and designers should understand that they don't get paid for these types of projects because Google is cheap, but because there's a huge community of artists who have been doing it for free for years. Go to a site such as DeviantART or WinCustomize; you can find thousands of themes and skins for various sites and applications completely free.
Of course, pro illustrators may (and often will) choose not to do this type of work for free. Fair enough. However, many of them will undoubtedly feel that if others agree to do such work for free (or for peanuts), it cheapens their profession and ultimately decreases their rates.
Illustrator Gary Taxali, whose post on art community site Drawger had drawn attention of other artists, many of whom agreed with him, expresses this sentiment:
"Google calls me and wants my work for their new search engine all over the web, the fee? Nothing. Editorial clients are slashing 1999s fees almost in half and citing the bad economy as an excuse. You know what? My excuse is that the economy is bad so you have to pay me MORE for an illustration. Hows that for an economic stimulus package?"
Skinning or theming is an odd beast; it's half art and half craft. Often, you can create a great skin by doing a bunch of technical tricks to the images and the code; you don't need to be a great artist to do it. It is therefore often considered a first step for a young artist on his way to becoming a pro designer, artist or illustrator. Very few actually succeed; most - like me - figure they're better at the technical than the artsy side of things and move to other types of work.
In short, this is just one of those changes the Internet has brought. A lot of people are willing to do certain things for free; you can't really blame them for that, can you?