It isn't for lack of trying. For many years, several of Intel's competitors unsuccessfully tried to push performance ratings over MHz. In the mid- to late-1990's, AMD's 5x86 and Cyrix's 6x86 lines used a PR- number. Cyrix's 133 MHz 6x86, for example, outperformed Intel's 133 MHz Pentium by such a large margin that Cyrix marketed it as a PR166+. Although AMD fought head-to-head against Intel in the MHz wars for several years, they returned to a performance rating scheme with the Athlon XP, and have continued to use various performance ratings in their current product lines.
Although Intel resisted for many years, even they joined the game eventually, once they started having difficulty pushing their clock speeds ever higher. At that point, they started adding more cores and optimizing the cores.
Unfortunately, there is still no industry-wide performance rating scheme--but, as others here have pointed out, the rankings provided by http://cpubenchmark.net are usually good enough if you don't have any special requirements such as I/O virtualization or hardware AES instructions.