The issue is that you can't put 10 of your 2 Gbps wireless links next to each other. You quite possibly end up with < 2 Gbps as interference kills your signals (unless you put the transceivers so far apart from one another that you sort of defeat the purpose). That said there are other wireless solutions that can get you > 10 Gbps over > 20 km already (not sure about 20 Gbps, but I wouldn't be surprised). The issue is available spectrum, i.e. you can't just setup the link, because the spectrum doesn't belong to you. Not a problem for optics.