'Isn't everything' is a huge understatement. It's a performance nice smoke test for a CPU in that if it isn't hitting the numbers you'd expect given the microarchitecture (issue width etc) you know there's a problem and due to the simplicity of the benchmark it shouldn't be hard to analyse and track down.

Though there's way more a modern high performance CPU needs to do. It doesn't do anything meaningful to stress the memory system for instance. Hitting good Drystone numbers is stage 1 in a long process to building a modern high performance CPU.