Coax runs in residential houses tend not to go through conduits, and often squeeze through holes in joists just barely big enough for the cable. Not to mention

Coax runs in residential houses tend not to go through conduits, and often squeeze through holes in joists just barely big enough for the cable. Not to mention that, in my experience, they also tend to involve splitters in the absolute most random of locations. Running Ethernet alongside it is rarely any easier than fishing from scratch.