Money. Landfills aren’t cheap to operate and burning also reduces transportation costs. Incineration ends up being quite profitable in isolation. Take stuff

Money. Landfills aren’t cheap to operate and burning also reduces transportation costs. Incineration ends up being quite profitable in isolation. Take stuff worth negative X$ per ton and turn it into a smaller pile of stuff still worth negative X$ per ton. Generating power is a useful side effect, but not the main reason.