Creating planet textures is quite a bit of work (especially if you want them to look good) and rendering a planet in an appealing fashion is expensive on top.
If you have a look at the EvE Online planets for example, which were procedurally generated last I checked: They look okay but not great.
Now with the amount of planets in STO (vastly less than Eve) creating nice individual planet textures and reusing them is a better approach.

Altho you have to consider that the time spent creating visuals for a nice, new planet could also be used to create a new outfit design or maybe fix the Diplomacy BOffs outfits...