Because it's monopoly. If you don't go to those who charge a fortune, you've got nowhere else to go, you'll have no pilots and the aircraft will be left to rot. And both the actual manufacturers and the training providers have their ways and means of keeping this status quo because it suits them well. If you're wealthy enough to run a Gulfstream V, you'll also find the money to pay for the sim.
The same goes for maintenance. You can service a C525 cheaply somewhere in Eastern Europe, but you cannot do that with a Gulfstream. There's only one maintenance provider in Europe, I think, the one at Luton. And they will charge you eye-watering sums for everything. It's just like driving a premium car. Why does a simple oil change for a Mercedes S Class cost a couple of times more than an oil change for a Ford Focus? For the very same reason. If you can afford a premium-class vehicle, you will also be made to cough up premium sums of cash to keep it running.