They are absolutely determined to squeeze every last dollar of ROI out of that existing coax cable plant by doing asymmetric RF channels for downstream/upstream. And not overbuilding themselves with GPON FTTH.
There's a limited amount of RF spectrum in a given coax segment and they bond it together in such a way that 85% of it is used for downstream, and some paltry amount for upstream. On a heavily loaded DOCSIS3 segment it can also be 12-13ms latency to the gateway which is an eternity in metro area networking fiber terms. So you end up with something like a 200 Mbps down x 15.5 Mbps up cable connection.
Obviously the abusive bait and switch customer acquisition and retention tactics are also a huge problem. Now they're doing additional bullshit like making special offers to bundle your cellphone service and cable together (Comcast is a MVNO on some nationwide cellular carriers), for a "discounted" price, so that it makes it more painful for you to switch your ISP service in the future if you're unhappy.
Eventually they will need to spend a spectacular amount of money to overbuild themselves because in some places the local phone company (centurylink, ziply, verizon, others) is building 1 strand of dark fiber to each house, running it to some neighborhood level splitter, and going 1G or 10GPON on it for service that approaches real symmetric gigabit to the customer. To individual single family houses.
Comcast is just the most obvious example of the problem in the incumbent cable operator industry.