Can anyone tell me why this is?
Keying QT 7 for "Pro" use turns on the editing/conversion features built into the QT structure embedded in Mac OS X. It does not, however, add any new codecs to your system's codec component configuration. "Pro" codecs are normally installed when "Pro" apps like FCP X, Motion 5, or Compressor 4 are installed or by the ProApps QuickTime Codecs v1.0.2 download. (Not sure if this download will install for QT 7 Pro alone without one of the mentioned apps.)
As to the fact that your codec component configuration is limited, be advised that under Lion and Mountain Lion operating systems, legacy codecs are, by default, turned off and that Apple cannot legally supply third-party copyright codec components which you, as the end user, must purchase on your own nor is Apple required to provide or support open source third-party codecs which may be available as free downloads.