Because:
there are so many variables when it comes to VPS hosting (noisy neighbors, SSD/HD, host server specs, number of VPSes on host, etc...)
VPS hosts like to use fluffy specs (vCPU) to sell their products
Many virtualization platforms (Xen, Parallels, OpenVZ, VMWare, etc) have their own quirks and optimziations
Different server configurations and optmizations have extremely high impact on the server's capacity under different circumstances (ngnix vs apache, fast-cgi vs mod_php, etc)
Most hosts have burst allowance..
I was wondering if there is any way we can objectively compare VPS hosts to another on raw performance capability and/or if it would be possible to develop such a tool?