> Because they'd probably have an easier time teaching a physicist to be more practical than teaching an engineer more physics.
This is somewhat debatable.
For example, Catia can be a harsh mistress, and her English is somewhat questionable, so organisations who have to work ~~with~~ for her are likely to just hire somebody who already understands her ways.
In more general terms, if we assume that all subjects are taught to an equal level (leaving out, for a moment, the thorny question of what that would mean), it would obviously make sense for the majority of employees to be selected from the course which most closely matches their job description.
However, not all courses are created equal, and the chances are that a better course will be more attractive, even if it's less targeted to the job description.
Indeed, some degrees aren't targeted at any kind of "real" job description at all, and might as well be called something like "badge of critical thinking skills, Bachelor level, Classics flavour" (though of course, a small minority of students will go on to teach).
In the end, it really depends upon the job that needs to be done, the nature of the organisation, and the state of the job market.
Some big organisations will have an HR department which hires to a precise specification, and such organisations will tend to be quite rigorous in defining the jobs on offer, not least so that HR can't pass the buck for any failures.
Other, usually smaller, organisations will be more flexible. They might well hire a few people will unusual skill sets in the hope that they will be useful for long-term R&D type purposes. Such unusual people represent a risk to the organisation, and therefore they tend to be very much the exception rather than the rule.