That's due to the DOCSIS standard for cable modems. They specced out more channels for downstream than upstream because of the limited bandwidth of copper and consumer priorities. With fiber there's an order of magnitude more bandwidth available, so the uneven split is much less (if at all?) common with the big backhaul lines between datacenters. For consumer fiber you'll usually get symmetric but for the most part it doesn't make sense as the vast majority of consumers just don't make use of their upstream bandwidth.