Hackers on the internet are lawless because they're in Russia or wherever, but somebody looking through your walls with wifi will be personally present in your street where he's at risk of being caught so it's a completely different kind of risk.
Cellphone jammers are probably a clearer example of cheap tech that can be used covertly but it's illegal and people usually don't.
Another important factor is that they aren't scalable because it takes a human a lot of work to hack only extremely local targets, not a program that can scan computers all over the world automatically.