Why are we either Left or Right - the denominator is that we are human.

Left and Right - why in politics, are people divided into these camps? We should not segregate ourselves like this. Some maybe just left of centre but are the as left as the left can be? Also as it's subject, what would the middle be? Would it be different in different countries (e.g., just left of centre in the US would most likely still be just right of centre in Canada) or contexts. Sure politically we are diverse and therefore a dichotomous variable such as left/right does not do justice. If we to use left and right it should be a continuum not just left or right.

The reason I bring this up is that I just read in a book "30,000 people 'disappeared' , and most of them on the left."
Why does it matter if they are on the left? Is this meaning that the people on the left are worth more/less than those in the middle or on the right? Would their disappearances mean less or more if they were from the right?

These people's disappearances are unfortunate so I'm just curious why the author would have the disclaimer that they are from the left. People are people, and it should be left at that (no pun intended).