To be charitable, I think it might also be because there could be situations things can't get to players, so giving them ranged attacks to avoid having to stand there being plinked away is done.
That is probably the most likely reason - teleport and range attack allows phase spiders to "skip" verticality of the level. That said, Druids can change into spiders, so spiders can climb ladders and such, so I don't think their mobility would be hurt that much.
Overall, for my taste Larian likes to make their enemies jump/teleport a bit too much.
