Diapers do eventually decompose but because diapers are made of plastic for the shell of the diaper it cannot decompose for several years.
Although new disposable diapers claim to be cloth and breathable they really are not breathable and only have a cloth backed covering.
The cloth covering is a thin piece of cloth material that is glued to the other part of the plastic of the diaper.
You can sometimes remove the cloth cover from some disposable diapers and it will have a plastic backing to it.
The diaper has to be made of plastic so that the diaper holds the pee in or the diaper would leak.
Plastic that the diapers are made of is the same type of plastic that most plastic bags are made up.
The plastic bag and the plastic diaper will eventually decompose but it takes several hundred years for the diaper to decompose.
Some types of diapers can be composted completely while others require the SAP to just be removed to compost.
To compost diapers they must be ripped apart or shredded up in a machine.
The diaper cannot just be placed into a compost pile as a whole diaper or it won't break down properly.
In most other disposable diapers that are not composting type diapers you can rip open the disposable diaper and compost the SAP in the diaper.
Then throw the plastic part of the diaper away.
Diapers that are advertised as composting diapers can be composted by tearing the diaper up or shredding the diaper up and placing all the diaper parts in the compost pile.
The diaper needs to be ripped apart or shredded for the diaper to compost properly.
You can also just bury the entire diaper in the ground if you have no trash service.
Or place the diaper in a burn pit or burn barrel and with a hot enough fire the diaper can eventually burn although it may take a few fires for the diapers to burn.