They have the lowest prices on the market for large purchases. The fee is negligible. Try buying such quantities elsewhere without KYC, you will pay much more.
They probably have a philosophy that holding Monero is inappropriate, considering the possible sharp drop in price upon delisting.
Their philosophy is that the amount of XMR in the balance depends on supply and demand. And that's all. It's been a while since someone came here to complain about how eXch calculates here and deliberately doesn't have XMR available in the reserves.
As you say, they have the best rates plus they are 100% KYC free, it's strange that they don't have unlimited XMR