I don't necessarily think that one is more complex than the other. But I think that physics and chemistry were mostly dealing with unobservable things, so they had to come up with abstract laws, rules and regularities of the universe. Which means that they had to rely on their creativity and imagination more to come up with abstract ideas. Things that you can't merely observe.
While observable things like people are well... observable. So they thought that they could just rely on things like statistics and simply note their observations, without bothering to come up with any abstract theories dealing with abstract ideas. Well they were wrong.
Saying that you can't speculate because it's not "real" is like saying you can't figure out how the iPhone works, you can only observe.