Hello,

The auto accounting option is a major role in AR, Why this option doesn't existing in AP.

Please let me know.

Thanks,Raghu.K

AP and AR works slightly differently.
AR was originally purchased by Oracle from a third party long time ago and AP was built in house. That is why they are slightly different species.
AP uses a different mechanism to achieve the same functionality as AutoAccounting.
You can use distribution sets to automatically populate the expense accounts. Payables liability account is defaulted from supplier site (which in turn defaults from supplier, which in turn defaults from Options).
You can use Automatic Offset Method settings in Payables Options to dictate how balancing segment value would be defaulted for payables liability account.