I agree that contributing to open source is a good way to learn many development skills, however:
Besides, open source is fun, and you probably have way more time and resources during University than you will later in life.
Not necessarily, if you are doing CS at a top university and aiming to attain the highest possible grades this requires a huge investment of time and effort to most people.
Also it is not like a 9-5 where you can 'clock off' there is always an uncertain feeling of 'if I put another hour into this , then that might just bump me up an extra grade'
Not to mention that if you just started programming at university your code is probably going to suck pretty hard for the first 2 years at least, to the point where no serious open source project is going to want your patches.
Ultimately if Universities believed that this was important to job prospects in the industry as a whole they would make open source contributions a required part of the curriculum.