As other comments have pointed out, there's a lot that contributes to this - it's a pretty low threshold, we understand better that it's actually GOOD to stay home if you're sick...

As a teacher until just last year, one of the things I think is contributing is a lack of consequences for the child and/or parents. Say a kid is chronically absent. What exactly happens to them? They might get some detentions (which of course they won't serve because they're already absent). But the teacher is still held responsible for catching them up when they return, regardless of how many days they miss. By the end of my career, it was no longer "What work should I do to catch up?" but rather "What are you, the teacher, going to do for me?". It's very, VERY rare that a kid has to actually repeat a grade; schools will do whatever they can to avoid that. In essence, I don't think kids (and even some parents) actually care about making sure attendance at school is as consistent as possible.

But there are a LOT of problems with our educational system, and this is just one sign that it's very much a failing system.