[MikeE]

There are exactly* 3 reasons:

1) Intellectual. Software is conceptual not physical and therefore harder to understand than mechanics. As so many have said above, it's easy to blame something you don't understand and may be slightly frightened of.

2) Number and complexity of interfaces. There is far more to the software than just the code a typical team's programmers write. All of these other software components have bugs, undocumented assumptions, edge cases etc. and are not well documented.
The mechanics just interacts with itself, the carpet, a ball and maybe other robots.

3) Mentoring. Many teams have a no software engineers amongst the mentors, so students don't get advice on reliable design, robust coding practices, or understand how to manage the central strength/curse of software: it's flexibility (e.g. through configuration management).

* rhetorical license #MA32145