Because if you want good support, such as OS lifecycles longer than a couple years, they're actually fairly inexpensive. RHEL starts at $350 a year to get you the same kind and length of support you'll get from a $300 MS subscription. Yes, the retail price for an MS OS is high, but if you're going to be doing server deployments, you'll be getting your software from direct resellers, who do provide much more reasonable prices.