[ 16/8/20 ]
Short answer is because information is increasing and models are becoming more complex. Now to get a reasonable understanding of biological evolution for example, one needs to be able to handle some very complex math, have a reasonable grasp of quantum mechanics, be able to deal with multidimensional probability vectors, and have a very wide range of experience of technologies and ecosystems and geology.
Most of what was taken as obvious facts 100 years ago is now seen as rather crude approximations.
Most practitioners now deal only in probabilities.
Understanding itself has changed significantly for most.