That's not really true. There's certainly knobs to turn, and almost certainly some available capacity to increase uploads, but coax and dsl are fundamentally asynchronous.
Coax has to manage the single (or few) headend transmitter vs many CPE transmitters, you get better multiplexing to the CPE than from the CPE; and while the newest equipment may be able to handle CPE uploads across most of the spectrum, older equipment had a narrower range of frequencies available to upload with. If there's spare frequencies and downstream capacity isn't using it, then yeah... if not, there's trade offs.
For DSL, everybody has a dedicated pair to the DSLAM/remote terminal whatever, but crosstalk between pairs impacts the signals and again, the DSLAM has more ability to mitigate that towards the CPE than the CPE can when sending upstream. Anyway, VDSL/VDSL2 are specified with asymetric up and down bandwidth, as was ADSL. Of course, DSL ISPs tend to cap sync rates based on profiles, rather than running the lines to their maximum capacity, so a lot of lines do have more upload capacity than is allowed to be used, and to the extent that's the case, it would just take a profile update to get more upload for many users.
DOCSIS (the Coax protocol) is asymmetric. 10:1 for docsis 3.1 and 10:6 for 4.0
So cable at least will always be a bit asymmetric. A lot of fiber deployments (GPON) are also but the bandwidth limits are much higher so it does not really matter.
