Precarious employment in academia: Why is there no labor union in academia? Many positions in academia are precarious, when it comes to employment common behavior in academia would never be acceptable in industry. This applies equally to PhD students as well as to junior academics:


PhD students are not guaranteed employment at university
after working there for 5 years.
PhD students are paid little considering their degree: In
Germany e.g. it is common practice for professors to only pay
    half-time posts.
It's common practice for young academics to work well beyond 40 hours even for PhD students that are
only paid half-time posts. Work regulations regarding maximum worked hours and weekends are often ignored.
Junior academics are unlikely to land an unlimited full-time job at a
university. Directly after graduation this is impossible and even after years of more precarious work as a postdoc only a fraction get unlimited employment.
With exception of CS and engineering very little universities care about providing an exit strategy for junior academics and PhD students after academia. (While change in industry is possible in quite a few fields, proper (additional!) training would be much needed.) 


These issues are well known. Fundamentally academia is producing too many PhD students and too little full-time positions (not just professorships) for their respective fields which causes much of the mentioned problems.

I feel like the passion almost all young academics and PhD students have for their subject brings a lot of naivety when it comes to employment conditions, which is exploited.

Why is there no movement inside academia trying to change these working conditions or a push to establish labor unions?
Why is there no labor union in academia?


Some universities have unions, others do not.  The scope of the unions varies.


  Why is there no movement inside academia trying to change this?


Many people support changing academia, so the premise of this question is wrong.  The reality is that change is hard, especially when the old system obtains lots of cheap labor.  


  how successful have they been in the past?


Unionization usually results in an increase in pay and job security, but the change is small.  

For problems 1-5, the solution is simple:  Don't get a PhD.  As long as many people try to get PhDs, economic forces will encourage these problems to persist.