Q. are not coffins haram? If so then why do we use them?

A. If the law of the country imposes upon you the usage of a coffin, then in such an instance you can use it. You must use the cheapest one, the one possessing the most inferior quality, the one that can degrade (decompose) the fastest. It is to be used out of necessity, and not personal volition.

To use a coffin voluntarily is prohibited.

Allah Certainly Knows Best.