Because too much of that is counterproductive. I have tried to do my homework on successful vs. unsuccessful Kickstarter campaigns, and the most important lesson I've learned is that "success" isn't "I got all the money!", it's completion of the project. And too often, artists (we often don't think things like this through) go a little nuts thinking up the bonus goodies they offer their backers...only to realize that they're having to spend more of the money they raised producing those goodies than they anticipated, and it's eating into the actual project.
Say, for instance, I offered a T-shirt starting at the $30 pledge tier. Well, to get a 1-color silk-screened T-shirt done, I might be looking at a unit cost of $5-$7 per shirt. Then there is shipping, which could run around $5 for US first class mail, or around $10-15 if it's going overseas. So right there, anywhere from $12-$25 of the $30 you'd pledged me to make my movie with would be going to print and mail a T-shirt! Five or fifteen bucks left over for the actual production? FAIL.
I want to promise all my backers that all of the money they pledge will go to the production of the film, and things directly related to the promotion of the film (the one-sheet, for instance, is a useful item for festivals and related theatrical screenings) when I am shopping it for distribution. Anything blown on trinkets and fannish merch is defeating the purpose.
I have also noticed that many people who come to Kickstarter look at the various pledge tiers, and think they must donate in one of those amounts only, and leave without pledging if they don't think they can afford it. This is why I wanted to make people who wanted to pitch in, but whose funds were limited in these days of belt-tightening, feel like they could still be part, by making the download available for only 2 bucks. Conversely, you can donate as high as you please, and decline the reward if you don't care about it.