We didn't choose that visual up front, we worked with a designer that chose that look based on the product (Minimal Analytics). The brand has defined the product over time and is very recognizable.
Whereas a flat css theme wouldn't have been unique or recognizable.
But I'm not going to change your mind ;-)
It's not unique or recognizable anyway. Minimal Analytics looks identical to hundreds of other sites.
If you were just going to settle on a generic, flat design, you could have used a free css framework and gotten the same result.