Why are bovines different genera if they can interbreed?
Species are not defined by whether they can interbreed, not even close.

In fact, there is no objective, scientific standard for what defines species, or genera, at all. What is a species is determined by little more than the "feels" of the individual naming it, or the committee approving it. 

And the same is true of the entire rest of taxonomic structure. It's not really science, it's just a way of organizing ideas that got popular, and in fact is under constant assault now by somewhat more scientific things like genetic testing. The hierarchy of the taxonomic system changes pretty much daily, and has no official structure or definition at all. Even the number of kingdoms is entirely up in the air.

Getting back to the specifics here, since there is no objective, scientific standard for what is a species or genus, it's no surprise that separate ones can interbreed.

And it's not like bovines are the only ones. There are many other intergeneric hybrids possible, like sheep and goats, wheat and rye, camels and llama, house cats and servals, marine and land iguanas, et cetera.

Perhaps someday there will be objective definitions to establish a truly scientific taxonomy. But for the moment, it's just whatever people in positions of power feel makes sense, at any given moment.

List of Intergeneric hybrids