Why are stars generally several light years apart except in binary star system, etc.?
Stars need a lot of gas to form.
Stars are born in nebulae. A nebula is a cloud of gas and dust which is very diffuse. When a nebula collapses under gravity a star is formed.
It requires a lots of gas to make a star. This means that the gas cloud must be large enough to have enough mass to make a star.
Effectively the formation of a star depletes the surrounding area of gas, so another star can't form close by.
It is possible, and indeed quite common, for two or more stars to be formed from the same gas cloud. This explains binary stars.
So, the reason why star systems are typically light years apart is that each star system is formed from a large diffuse gas cloud and the formation of the star depletes the region of enough gas to make another star.
One exception to this is in open and globular clusters. This is where a dense gas cloud condenses into a number of stars in a short period of time. In the core of such clusters, stars are often less than a light year apart.