Duplicity Vs. Singularity of Mammals Organs
It's just how we evolved. At some point in the past, a distant ancestor of ours had two lungs, two kidneys, one liver etc.( maybe then there was a pretty good reason for this). We evolved following that pattern and now we ourselves have two lungs, two kidneys, one liver etc.

Snakes, for example, have just one functional lung because their elongated form doesn't cope well with having two full-grown lungs. Humans on the other hand have a body shape that works very well with two lungs, so we didn't have to sacrifice one lung.

There are other examples of different organ distribution, like the octopus which has 3 hearts. Usually the more different the internal symmetry, the more distant the common ancestor.