On the contrary: the hard sciences are easier to learn by yourself because it takes more self-deception to fool yourself into believing you understand something you really do not. When you do a math or physics exercise and get 20 for an answer, and the answer back of the textbook is 3, it takes a heroic amount of self-deception to convince yourself you understand the exercise.

Then again the hard sciences are less likely to be taught badly because falsehoods about the hard sciences are easier to detect, so if you are more worried about false teachers than about arriving at false understanding through self-study, it tends to make sense to reserve the more difficult-to-get-right subjects like philosophy for self-study under the theory that if you want a job done right you should do it yourself rather than trust a bureaucratic educational establishment to do it.