An analogy...
Here is an analogy to the question...
There are so many cookies in my favorite bakery. It would be great if I could take a free bite out of every one. I would be able to tell immediately if I wanted the cookie after just one bite. Why don't bakeries allow this?
As you can see, the question looks a bit strange when it is applied to something that people are accustomed to paying for. As with cookies, time and resources go into developing games, and developers need to recoup those costs. My husband and I developed our first game,
Whack 'em All! at night every evening after our children were in bed for several months. We had to spend money on hardware, software, iPhones, and music and sound effects. Even though we have created a high-quality game that has gotten great reviews, we have not yet been able to recoup our costs or our time. We are charging the lowest possible price allowed in the app store (other than free), and have a video demo of our product so that people know what they are buying.
We've considered creating a lite version, but as others have said, it's hard to justify the resources that would go into that, when those resources could be used to enhance the existing game.
Thanks for posting this question; it can be hard to see both sides of this issue. The fact that you've asked it helps me understand our customers better.