This one may not be appropriate. See the back story in the databanks on how all of the races except the Terrans probed the galaxy before the Terrans came on the scene with their hyperdrive invention.

Playing as any race but the humans means that you do in fact know where all the planets are, as the Stellar Cartography technology places markers on the minimap showing the location of every planet in the game without having to explore that region of space. Humans are the only faction which lacks that technology at the start of the game, meaning that you have to explore to each star to determine if it has planets and whether or not those planets are inhabitable. Nonhuman factions can send explorers directly to systems which do have planets.

I also disagree that the other races should have a fully explored map, as presumably a large percentage of the exploration was done a very long time ago, as it was carried out by automated probes traveling at less than the speed of light. Even if the communications systems were instantaneous, the probes would have taken centuries to cover even a moderately-sized map, and it isn't that likely that the various races have had probes continuously monitoring the explored systems which were in some way interesting. Moreover, we know that at the very least the Arcean and Drengin Empires have been exploring for a minimum of 70,000 years, as that is the amount of time it took the Drengin to send a stargate to Toria, and this appears to have taken place after the two Empires cut off contact with one another. As such, it is very likely that the information held by the various factions on many systems is extremely dated and may not be particularly reliable for determining good colonization prospects.

It depends on which game you are playing. In DL and DA, only the Terrans are missing Stellar Cartography. In TotA, the Altarians and Krynn are missing it too. However, I think, that this is an oversight and not intentional. Well, at least in case of the Altarians. It could be argued, that it is fitting for the Krynn (and Thalan) to not have it, because they are new to our galaxy.