Why haven't consoles multiplayer combined?

I've thought about this for a long time, why can't games such as the Madden franchises, Call of Duty, Battlefield, any genre you think of that has cross-platform games (even branching into the PC world) have multi-platform online play support? I have friends with Black Ops that have Xbox'es that I can't play against.

I understand each Console/PC have different operating systems and internet configurations etc; However, in the near future Diablo 3 will be available on Playstation 4. Which leads me to think, will it be able to play against those who log in via a PC? And if it won't, why not? Login services that can be installed such as Steam could be an easy workaround for it, and most consoles would be able to support like-programmed software as such. There is no reason not to expand a userbase online.

Yes, there would be issues with who pays for the servers/bandwidth load, what percentages from which specific platform pulls how much latency, etc, etc. But, expanding a console to play more like a PC makes the absolute most sense. Is it inevitable? Absolutely. Will it happen in the next 5 years? There is no way that it won't. But why hasn't it happened already? I wan't the bridges to be gaped, and the incoherence and incompatibility to be gone.