An ASIC design would be incredibly expensive to only build a couple of. Mass production brings the cost down. If I had to guess, they're selling these because once they hit, difficulty will go up and mined production will go down. Transaction fees that pay the rewards are a fixed value. Your share of that value depends on your ability to be better than everyone else. Once these hit, the "average" will rise. Everyone spending money on expensive ASIC machines are doing so just to keep up with the average.