CATV was designed as a one-way pipe. There are filters everywhere in the network that block inbound traffic because there wasn't much of a reason for cable boxes to need to communicate with the wider network which meant that the bandwidth on the cable that was actually usable for upload bandwidth was orders of magnitude smaller than downloads. As Comcast basically rebuilds their network (fiber to the edge), they're bypassing and removing all of those filters which means that there's plenty of frequency for uploads.
Designed, maybe, but it's not like the cabling is directional.
I thought one of the reasons why DOCSIS was straightforward to rollout was because they only really needed bi-directional amplifiers to do it. Though in the early days of cable internet, some setups used a telephone modem for your uplink.
And if you already had Hybrid-Fibre Coaxial infrastructure (which was rolled out for noise reasons in the pre-data analog days) you were already a step ahead of the game in terms of segmenting nodes/worrying about CPE noise.
Upload is crap because they have to dedicate channels to it at the cost of download channels, and upload has more overhead because of the coordination required.
70s/80s cableTV companies basically won the lottery by being able to re-use their existing last-mile plant for high-value data. Telcos not so much.
