It is simply impractical to park a Donosev survey ship and its crew in orbit around a planet for a few years while it does the surveying. Unmanned satellites are the more economical and reasonable option. However, sat surveys have limitations too. At high altitudes, a sat with optical or visible light cameras can see a lot, but not in any useful detail. The standard Traveller world map is a good representation of the problem.
Look at the map of Earth as presented in the game Invasion:Earth. The area of the continental United States is covered in four hexes. Three thousand miles in four hexes? Each hex is 713 miles in width. There is, for all practical purposes, NO detail of the surface available from that map. Even the hex that represents the Rocky Mountains is so vague as to be almost useless. Humans don’t live and act at that scale. The continental US has almost every type of terrain found on this planet, from swamps to river deltas to deserts to mountains, foothills and coastal plains. None of that is depicted on the I:E map.
See what I mean about lack of detail? Can you tell where the Appalachian mountains are from this photo?
That is the kind of representation that a high orbital satellite provides. It’s quick and easy to do but provides the most meager information.
The reverse is true as well. A camera sat might focus on a single square kilometer, or 10 meter square of the surface, and produce a very detailed image. But only for that one square. What is adjacent to it? What terrain must be crossed to get to it? How long did the sat take to find that one spot – a week, a month, years? Satellites orbit, and for this reason, are able to look at any one spot for a limited time each time it goes around. Maybe not much changes from one orbit to the next, but maybe something does.