Why don't GMs tend to play simpler openings?

Like, why instead of playing the London or something simple, they go for like an Indian Game or Slav Defense? Or instead of an Italian Game they have to play a Vienna or Scotch? I just don't understand why GMs do this when simpler openings are sometimes able to develop pieces just as fast and get a good position just as fast, if not faster, than complex openings?

Sometimes they want to play an opening that leads to complications where both players have winning chances.

They play chess psychological transpositions. 1...Nf6 is a good example. That could go different ways. They'll transpose through two or three possible openings before getting on with it.