What drives indie RPG publishers to create products? Matt Finch, Swords & Wizardry creator, said this:

When you post up something on the internet, whether it’s a free resource or not, there will be several types of responses. Many people will type “Cool!” whether they’ve read it or not. These people, god love ‘em, are wonderful even though you know that they’re supporting you for what you’re doing, not for the quality of that particular resource. You can’t draw any particular conclusion from “Cool!” but without these guys I believe there wouldn’t be any free old-school gaming resources on the internet at all. The reward for producing a free resource is entirely ego-driven; you want people to see your work and enjoy it, and if there is no response to it you will assume that nobody read it, or perhaps that they did, and didn’t like it. Almost certainly, if you get a zero-reaction, you’re not going to bother to do all that work a second time. Source

Matt Finch goes on to say:

This, I think, is one of the major arguments in favor of for-profit publishing. The money might not be financially significant, but it’s psychologically invaluable as feedback. I definitely think it keeps people producing more resources than they would if they relied solely on the internet for the pats on the back that are so vital to a writer’s continued willingness to put pen to paper. Source

Rachelle Gardner, a Literary Agent, had this to say about writers wanting validation for their work:

It’s perfectly natural to want validation for your work. We all want our words to be read, and we want some kind of proof that what we wrote isn’t dreck. We know it’s subjective, but still, we crave the affirmation. Musicians want people to connect with their music. Painters want their work appreciated and enjoyed—and purchased. Those of us who write blogs want validation through our hit counts and comments. And most people who write books want that stamp of legitimacy that a traditional publisher brings.

I hope you don’t feel apologetic for admitting you want the validation. I think most writers, past and present, want this. Great writers like Hemingway and Steinbeck seem to have craved it. Most present-day successful authors admit to it. If you care deeply about your craft, your words, the message you’re sharing with the world, how can you not care about the world’s validation?

The method of getting validation is probably going to change over the next few years. For some, it won’t come through a traditional publishing deal but perhaps through more direct means—people buying your books and responding to them. For now it’s still reasonable to hope for a traditional publishing experience, but I also think it’s helpful and important to recognize your need for validation and begin to explore your assumptions about how to attain it. Source

So it seems to me that indie RPG publishers want affirmation for their creativity. We’re social creatures by nature, and publishing is one way to gain that affirmation we seek of our creative work.