How else would humans be biologically driven to stay together if not for an emotional investment?

Financial reasons, perhaps? I think it's common sense that in a species as evolved as humans (in contrast to lower members of the animal kingdom), the type of economic environment in which we live, as well as the related level of productive material development in any given human society, likewise influences the prevalence of monoamory vs. polyamory as a cultural practice.

It can also be argued that in regards to humans, spiritual factors (or the lack thereof) play an important part, since the prevalence of monogamy or polygamy appears to be greatly influenced by whatever type of religious tenets one does or does not follow (if any at all, of course).

So human behavior along these lines may not be solely based upon pure biological and/or emotional factors alone.