There are many reasons, but the classic explanation is that professors (especially at research universities) are picked (and compensated) for research ability versus teaching efficiency. (In general...caveat hawks.)
Probably a secondary reason is that mathematics tends to be a field with a high emphasis on logic and precision. However EDUCATION is more of a practical art (with aspects of psychology, leadership, tribal lore, efficiency, trade-offs, decisions with imperfect information, etc.) that are very different from Euclidean proofs. So, it would not surprise me that this issue occurs a lot in math, as a field.
The one counteracting tendency is the need for large amounts of service teaching for engineers and natural and even applied scientists (e.g. econ). This has some forcing to drive more effective teaching. On the gripping hand, if you're in a majors course...:-(
Note that none of this is new to the world. You might also ask many other questions about non-ideal behavior (in many places, other than math ed). Part of becoming an adult is realizing there are a lot of imperfect people/places out there. Don't get too exercised by it. Just do what you need to do, to optimize things for you. Find out which profs are good, buy Schaum's Outlines, etc. Maybe too late for you (but not others), but one attraction of "liberal arts schools" is more emphasis on teaching. You can always "go to Berkeley" for grad school.