The lack of concern with planets or resources is quite possibly a red herring. The Shivans appear to be operating under plans that are too short-term in scope for either of those things to be valuable; exploiting either would require developing infrastructure and the like, which takes time.
The presumable ultimate Shivan plan in FS1, involving the glassing of every world in Terran-Vasudan space, would take perhaps six months. Unless they were prepared with prefabricated factories, mines, and a large noncombat fleet just laying around, this isn't really enough time; furthermore the Shivans didn't see a need to reinforce their existing expedition, although there was apparently a source of supply somewhere beyond Ross 128. (Which makes what happened to said source one of the great unacknowledged mysteries of FreeSpace.)
The ultimate Shivan plan of FS2, which we saw executed in the Capella supernova (assuming the supernova was intended) and whatever caused it, is even shorter than that in scope.