The difference here is accessibility.
There's a number of public ways (which could be automated) to find out information like people's home addresses, but generally it's pretty difficult to figure out how (provided someone isn't just publicly posting their address).
Online harassers are often deterred simply because it takes too much effort to find out the information to do more harm.
Doxxing takes some amount of know how, and that's the only thing preventing it from happening more often.
