There's a lot of artwork that would go to waste when joining the gaul and brit civs again. There are village-phase buildings like houses and barracks made specifically for one of those two. So if they shouldn't be available as a pre-game choice, we'd miss that artwork.
When I joined, the situation was actually backwards. A scenario could use the celt or hele civ, but it couldn't be selected for random maps. Of course, that created problems because the celt and hele civs were never properly tested, and were sometimes released with bugs. It's because of those bugs, and the exceptions they always needed in the code, that the generic civs were removed.