Wasn't always this way -- see this historic reconstruction of a Baroque orchestra https://www.youtube.com/watch?v=fNqJ8mED1VE (Handel, Music for the Royal Fireworks on period instruments)
Down on the lowest level of the stage we find a small string orchestra, but then behind it on risers there are something like 18 oboes(!!), 10 bassoons, 9 horns, 9 trumpets.