RHEL source code is freely available under the GPL (GNU General Public License) for those who want to compile it themselves, but the actual finished product costs money. Yes, there is CentOS, a free-to-download clone of RHEL compiled from the source code by CentOS developers. But Red Hat charges a premium for RHEL because it's (theoretically) guaranteed to work—Red Hat and third-party software vendors make sure that applications running on RHEL are not broken when the operating system is updated.