I think there are a few reasons:
1) I can't explain why, but, unlike physics or math, distillations and commentaries often miss key points and insights, and not uncommonly badly misrepresent works. Reading one then the other makes this clear. It's also the case that the canonical works are often (not always!) well-written, so difficult to improve on—possibly this is a side-effect of how philosophy works, as a field.
2) (I think this one's very important) Historical works of philosophy (which, see above, also have a tendency to be quite good for what they are) are, in themselves, a bit like exercises in math. Skipping wrestling with (at least a good number of) them is like trying to learn multivariate calculus without having yet put pencil to paper and worked through any exercises, for any kind of math, your entire life. In math, you have to do the work, to learn the material. Critically reading touchstone works in philosophy is philosophy's equivalent of doing the exercises, to a substantial degree.
3) The way philosophy has developed is kinda like rap: a lot of it won't make any damn sense—or, worse, will seem like it does, but actually you're getting entirely the wrong idea—unless you're familiar with the work of the person this track's responding to, and then that one is also part of a larger conversation, et c. Maybe that's a bad way for it to have developed, but that's what we've got.