atop can only monitor network io with the netatop kernel module installed. In theory it could be a separate package, but I'm not sure what the point would be since this is the only software (that I'm aware of) that uses the module.
atop has a big advantage over other system monitoring apps IMO when monitoring network io, since the kernel module allows monitoring without a heavy CPU footprint that the userspace equivalent has. (Take for example glances, which has a very nice looking interface, but can be a real CPU hog)
@Ruben Martin: One point in netatop being a separate package would be making kernel rebuilds more straightforward. "emerge @module-rebuild" would capture this as a module, and would avoid having to rebuild all of atop just to build the module. :)
Right now, it is kind of sad that we can't take advantage of all of atop's integrated goodness with network stats per-process.