Lectures often don't have citations. It breaks up the flow. Basic text books often don't have citations. Citations by themselves are also meaningless.

The ideal of science that we are taught boils down to one premise and one premise only: as part of a scientific statement there will be a recipe. By following this recipe we can replicate the findings of the author.

This ideal is met less and less the more complex and expensive our science gets.

(Thanks for the support and clarification btw!)