Why are there several languages in the world?

Firstly, because we are not even sure that there was monogenesis of language. That is, we are not sure whether language originated in a single contiguous community of humans, or multiple communities.

Second, because like all social phenomena, language is a dynamic system subject to change through conflicting factors. Change at an individual level is moderated through the pressure to retain intelligibility within a community. But if two communities are distinct, there is no longer any pressure for their languages to remain intelligible to each other, and they will end up evolving independently and diverging.

Third, language is one of the primary vehicles of group identity. There is a strong motivation for groups to ensure that their language is distinct from that of rival groups. There is the example I read somewhere of a language in Papua New Guinea which historically seems to have changed all its p’s to k’s. That change makes no sense phonetically, but it makes a lot of sense if you are doing your damnedest not to sound like the next village down the road.