Why do some universities not provide marking schemes for mock exams?

The materials provided to students should benefit their learning. It is not clear, how the marking scheme would help student to understand the actual material. Knowing the marking scheme can help a student to prepare to a particular exam without learning all the curriculum (e.g. by focusing on what is assessed). This is known as surface learning, and is often considered as bad practice.

All material has a cost/benefit ratio.

Writing a marking scheme suitable for distributing to students can take quite a while (normally mark schemes are designed for lecturers, so some parts can be quite brief). Students will query cases where they feel they were one or two marks too low, even though the exam was only a mock paper so the result is not really important.

So then the question is, what is the benefit?

In addition to the other excellent answers, there will always be some students who will assume that the marking scheme for the final exam is similar to the one for the mock exam no matter how often you told them that this is not the case.

But since the questions in the final exam will obviously be different, the marking scheme there will most likely be different, as different errors can be made by the students and must be taken into account in the scheme. Also, some parts of some questions may be trivial in the final exam, and you may want to assign a fewer fraction of the points for those.

So you will have to convince some students who will try to argue that they should get more points using old marking scheme and claiming that it would be unfair if the marking schemes differ substantially.

As a lecturer, you don't want to spend you time with such discussions.