Not exactly. It's true that a coax cable can, over distances of hundreds of metres, carry up to a frequency of 850 MHz at a fairly low attenuation and, at greater attenuation and/or ideal circumstances, up to 1.2 GHz in a useful capacity. This may translate, as per Shannon-Hartley and QAM into a raw bandwidth of multiple Gbps.
However, the topology of most FTTLA deployments doesn't imply that any individual customer is guaranteed this speed. In many deployments the LA - last amplifier - will be shared among hundreds of customers. In some cases, it may be only dozens. In some, it may be in the low thousands.
While the TV multiplexes are always going to take up a huge amount of space, depending on the deployment easily more than the majority, it's also becoming more common, for example, to dynamically switch multiplexes onto the spectrum on demand only.
If you have a healthy contention ratio, quality coax, few people on your LA, it's a good time of the day and you have a sensible provider then yes, it's possible to get a Gbps over coax. But the stars need to align to get that magical moment when with GPON or especially 10GPON it's basically a guaranteed thing.
Comcast isn't rolling out Gbps speeds because they have a monopoly power and want to artificially constrain total bandwidth (well, they kind of are but in a different way), but because it would imply rebuilding their FTTLA deployments deep into the field to basically take them halfway to FTTP/FTTH.
They're not offering Gbps because they're a monopoly and hate selling bandwidth first, they're not offering it because they're a monopoly and hate investing money in infrastructure. After that, it's definitely the hating bandwidth thing.
If it was the former, they'd be able do a far better job competing against municipal broadband deployments and Google Fiber than they are in everyone's mindshare.
