DisplayPort 1.2 cannot even do 5k at 60Hz at 10 bits per channel. You might be thinking about DisplayPort 2.0, which is not yet widely supported in video cards or monitors, and even then it needs the (essentially non-existent) UHBR 20 data rate to get to 120Hz.
HDMI 2.1 does support 5K@120Hz@10bpc but requires DSC as you say, which is not very commonly found either.
(Note that unlike what a sibling comment states, DSC is lossy. Some argue that the less is not noticeable to the human eye, but not everyone agrees)
It's not commonly found on monitors. DSC is included starting with DisplayPort 1.4, and a good portion of monitors even today still ship with DP 1.2
Nvidia and AMD have not been shipping DSC with HDMI simply because HDMI included DSC starting with HDMI 2.1, which only the current generation of graphic cards and very few monitors support.
HDMI 2.1 does support 5K@120Hz@10bpc but requires DSC as you say, which is not very commonly found either.
(Note that unlike what a sibling comment states, DSC is lossy. Some argue that the less is not noticeable to the human eye, but not everyone agrees)