I think as humans we mostly and naturally, perhaps even subconsciously, look for patterns. I think some of us, depending on various things including personality, can be drawn towards and fascinated by patterns more than others. “You reap what you sow” might be part of that. However, the outcome of you reap what you sow might not necessarily be a given. It seems to me to be a ‘cause and effect’ phrase, and nothing more.
But saying that, any cause might not always have the same effect when repeated, and therefore in my mind, the phrase “You reap what you sow” may not be accurate all the time, especially when introducing the variable of ‘circumstances’. With that in mind then, I don’t see "You reap what you sow" as an absolute. It’s more of a concept, but a concept that resonates in different ways with different people. I don’t think I would be able to relate to someone who sees it as an absolute.
As for ‘Karma’,
*sigh…* 
this seems more of a spiritual thing to me. I usually say I’m the most unspiritual person I know, yet interestingly, some others disagree with my assessment of myself. Anyway, I think to myself, where is the scientific evidence that Karma exists beyond a concept or philosophy? I have never used the word karma when seeing something happen to someone. I just can’t bring myself to use the word that way, even if I tried. A 'brick wall' comes up in my mind if I think about saying it. It just doesn’t sit well with me. I don’t see the word karma and the phrase 'you reap what you sow' as being remotely the same thing.