Because I'm a fat ass and they'd probably compress the whole way down when I sat on it.
I know little about them but my son's Giant Boulder SE has variable compression, so you can stiffen it up if you want.
Another comment on value - people spend a lot of $ to shave a few ounces off the weight of a bike, but this is foolish if you the rider are carrying extra weight. There's a lot more weight to be shaved off the typical rider than any bike.
