I think the short answer is three simple facts :

1. eye candy sells
2. they are businesses not amateurs … businesses don’t do things that cost money
3. perceived value matters

It should be noted that with easier access to high end publishing even amateurs can produce better looking books (lulu.com being one such example).

And do not underestimate the amount of work that goes into a (good) rules set (or a mini for that matter).
You can’t charge 50$ for a set of 5 handcopied black and white A5 pages, because no one would believe that it required months/years of tinkering to get those rules to that point.
You can charge 60$ for a 200 page full colour A4 glossy hardcover even if it is filled with errors … because the perceived value is higher.

I love the full colour ‘gimme all the info’ books, because when I am new to a system I need as much info as possible.
How do I paint the minis ? What are they supposed to look like ? What’s the story ?
I also like having a short ‘quick start’ booklet that gets me the essence of the rules and how to play the damn game, because it makes learning easier.
As such I adore games that offer both and I’m not ashamed to admit that I am willing to pay for that privilege.

The one thing I don’t like is how some companies have carved up the game into so many additional ‘essential’ books & gimmicks that it becomes a chore just to get everything in one place. That’s like selling me a hamburger and charging me extra for all the bits that make it more than a bit of meat in a bun.

As I’ve said on these forums before when I first saw Battlegroup I was not a fan. It looked too much like a boring studybook that I’d grown to hate at school. The Flames of War books were (and still are) dead sexy.
Battlegroup has grown on me … but it needed time.

So yeah .. sexy pics do sell and are essential to this hobby if we want it to grow.
I don’t mind, because I love my collection of ‘useless’ books as much as I love playing games.