Treating bullying as everyone's problem reduces incidence in primary schools

I would guess it's a combination of "nobody sees the first hit" (since your attention is elsewhere, of course) and that bullies get quite good at testing boundaries and thus know how to avoid detection.

But, really, it's truly frustrating that as I child I was bullied relentlessly, and when I finally took my parents advice and stood my ground, I was expelled from school (due to zero tolerance). Those bullies continued to torment some other kids, of course.

This is far from an uncommon situation, over the years I've heard many more scenarios like this.

Some tolerance and considering circumstances is actually the sensible way to handle most anything. But that sounds like being "soft on crime", and the PR side is usually more important than the actual problem.