The diagrams of the spectrum bands are wild for me (coming from the RF world) - in that world, a 2GHz channel that I'd used in some systems was considered ridiculously huge, but here in fibre the 'small' channels are 50GHz!
People really don't get the enormity of the difference - when there were policy debates in my country about rolling our new fixed line infrastructure there were literally people saying "but won't all homes and businesses just be able to use wireless in the future?"
My armchair guess is that because traffic hitting the cable is already serialized in some way that larger channels make sense? Of course, those large channels could also be multiplexed in some way and most long-range lines run DWDM/OTN, so I'm just as likely to be talking out of my ass.
I think the point is that fiber can have wayyyyyy more total bandwidth than any wireless technology, so you can afford to make your channels much larger (with more bandwidth). The only reason to make a channel smaller is to make room for a different channel.