Because those synthetic tests are easy to compare to other processors.
And putting this CPU in "real world tests" because it loses in benchmarks pretty bad wont magically make the new FX into some monster CPU. Synthetic tests indicate performance in different situations using different parts of the CPU, so if it runs really slow in all benchmarks (hmm, ever wonder why they are called the benchmark?) they are going to run slow in other applications as well. You will only find 1 or two situations in very specific environments that will go against this, but that isnt the real world either now is it?