There's no limitation of coaxial copper cable as a medium that would require it to be asymmetric. So it's not about the physical medium per se, but about how cable TV companies (DOCSIS cable modem ISPs) have designed and built-out the rest of their infrastructure over the last 50+ years.

Cable ISPs' cable infrastructure carries a lot of baggage from the Cable TV days. Cable TV, before DOCSIS cable modem internet service became a thing, didn't need much upstream bandwidth. All the upstream was needed for was to authenticate the cable descrambler set-top boxes, authorize occasional pay-per-view transactions, and maybe do a few minor "interactive TV" things, like letting you check your cable bill from your descrambler box.

So typically, only frequencies from 5MHz to 42MHz were used for upstream signaling, while all the frequencies from 54MHz to 1GHz were used for downstream. So the upstream bandwidth was only about 4% the size of the downstream bandwidth.

Because they choose to place the "split" between their upstream and downstream frequencies around 42-54 MHz, all their filters and amplifiers installed on all their utility poles and equipment boxes and "headend" facilities are all designed around that very lopsided split. Also, every DOCSIS cable modem and cable TV set-top box and "cable ready" TV tuner are also designed around that split. So moving the split to up to a higher frequency to allow for higher upstream bandwidth would be a huge, expensive undertaking.

By the way, I'm speaking about North American standards/conventions here; Europe is a little different, with a slightly higher split. I should also note that since the cable operators' infrastructure was built around sending the same TV signals to all houses, they basically just run one cable to your neighborhood, and then use passive splitters and dumb amplifiers to split that one cable to all the houses in your neighborhood. So you don't have a dedicated coax cable directly from your house to their headend equipment, so they can't change the frequency split on a house-by-house basis.

Under current DOCSIS standards for how the upstream signaling is done, the most bits-per-second bandwidth they can squeeze out of their limited upstream RF bandwidth is only somewhere around 35 Mbps.

That's why you can't get symmetric DOCSIS service above about 35Mbps, and why even if you get gigabit downstream service, its upstream is only going to be 35Mbps or below.

If you call for business service from a cable ISP, and ask about packages with upstream speeds above 35Mbps, they have to build out fiber optic infrastructure to your business because they just can't do it with their existing coax cable infrastructure. Not because coaxial copper cable has a limitation, but because all their equipment up on the utility poles between their headend and your business was designed around a very lopsided division of upstream vs. downstream frequencies.