Why are high end bmx bike so expensive

The simple answer is because there is demand and people are willing to pay that much for them.

People pay for the name, nothing more.
I own an OLD K.H.S. cost a little over 400 bucks.
It got left out in the rain for years, got ran over by a city bus, has been dropped, dragged, and mishandled. It has outlasted Cannonballs, Treks, Giants, and Bianches. Some of these bikes are well over 2 grand and are now retired.

They're reliable in rough weather and terrain, lighter to drive, hence its a pleasure to ride them, you basically flying on air.

I rode a custom schwinn back in 83 and it cost almost 900 bucks.

Really high quality parts and stuff

Because there are people who will pay that much. Based on the actual labor and material cost the price should be half that but fools and their money ...