That's because a lot of people learn about the Bitcoin mining benefits, which leads to a huge competition in the space, and only people with the best computing power will be able to mine profitably. Thats why operating Bitcoin mining with computer's graphics card and central processing unit will only lead to wasting of time, and energy unless you wanted to mine some shitcoin at the early stage.
If there is also much competition in the mining rig factory creation, the price of the miners would have been different. In the meantime, use Whattomine to calculate the profitability of the coin you can mine.