Because all high frequency trading does is inflate the cost for those of us who do invest in the "old-fashioned" buy-and-hold manner.
I heard it best described in this way: There's a hot new gadget that's being released today, and you *really* want to go buy one. Unfortunately, as you're walking down the street, some hedge fund investors see you coming and quickly jump in front of the store milliseconds before you get there to form a line at the door. The store opens, the investor at the front of the line buys ALL of the gadget inventory. He then turns around and sells all those units to the guy behind him for a small profit, who sells to the guy behind him for a small profit, who sells to the guy behind him, etc.
Eventually, they get back to you, but now if you're going to buy that gadget, it's going to cost you some significant percentage more to purchase for actual use. And you don't really have any option if you're going to buy one, because every store selling the gadget has a pool of financial sharks circling the entrance just waiting for another "traditional investment" sucker.
In the end, the store doesn't benefit, since they still only sold the item at the normal price, and you don't benefit because you just got your price jacked up. The only beneficiaries are the HFT scum who have played the system in such a way to artificially inflate your costs to their own benefit while adding absolutely no actual value to the product as it passed through their hands. This DOES impact you, because the more of your investment that gets siphoned out by the hedge funds, the less you have left to actually invest in the original stock.