Because you did not build the kernel 'CentOS' way:
http://wiki.centos.org/HowTos/Custom_Kernel
Also, if you are building from a vanilla kernel (kernel.org), you may want to look at kernel-ml from ELRepo.
Ok - is there a way to build "just a module" that disables the PAE code? I'm running on a smaller CPU.
Thanks,
Jerry