You are comparing a desktop to a mobile computer.
You pay a premium when you need mobile computers because the components have to be smaller. A desktop does not have that same restriction so components are cheaper.

Desktops and mobile computers perform different functions.
Would you carry your desktop to a meeting?
Would you carry it out of the office?
Would you use it in an airport while you wait for your flight?
Comparing the two devices by price is absurd.
I can buy 200 Pez dispensers for 500gbp but if I need a mobile computer, I am not going out to buy Pez nor will I buy a desktop.