Its cost of support and manufacture.. small numbers cost more and linux machine sales are very small. That is why the thin client costs more than a PC anyway, A more powerful notebook with a screen is less than the thin client with a screen. Solution.. buy the notebook and put linux on if you want or keep windows if you want or both.
If you want to do something about it organize a few tens of thousands of linux users to club together and specify a machine and buy them at a reasonable price. Intel did just that with ZIF sockets.. they cost $40-$50 each once.. nobody bought them which limited Intel's upgrade plans.. so they went out and guaranteed an order of a million ZIF sockets just to get the price down to a few $.