I do not agree with the thesis stated that "software engineering is slippery because humans are unpredictable". Maybe it is harder because it deals with humans, but good and solid research is possible, just look at areas like information retrieval.
The field of "human sciences" is slippery.
Economics, as a mere example, aims to be a "real science" and there is "good and solid research" but, nonetheless, the results are under our eyes.
I will not try to ask why this happens, many epistemologists have tried to.
umm, hehe, yah testing and high reliability, yah yah but umm - mathematics is a different kind of predictable...it's that kind of predictable where even if you don't know anything else, is it still true? yah. if we were in a different multiverse, where the laws of physics are varied in some ways, would it still work? yes. deduction is a beautiful thing.