Why are landfills like this still a thing? Why not burn everything and only dump what's left?
And that's before accounting for people illegally tossing things that contain heavy metals (such as batteries).
The only way they can seem to make a buck is to charge high rates to dispose of 'medical waste' that helps with costs.
90% : Sweden or Japan ? https://sensoneo.com/global-waste-index/
Medical waste is 'biohazard' waste (think blood, guts, needles, etc) So they can't just throw most of that in a landfill, it has to get burned. They charge a premium partly because they can, and partly because most of that is from outside our county.
We do burn trash, but as the other reply said, it is expensive because you have to somehow account for all that.
[1] I don't know if that's true.
I understand places like Japan wanting to find alternative means
It is possible to design burners that ensure a complete combustion, but they are much more expensive than simple burners.
Upon a complete combustion, some things may still release volatile acid oxides, e.g. oxides of nitrogen or of chlorine, but those can be captured.