Why a record cannot be updated in after trigger?

Apex triggers allow you to choose to run code at specific points in a database operation where the semantics are well defined:

before trigger: before the database has been updated after trigger: after the database has been updated So by design, if the platform allowed modification of the record in an after trigger, the record cannot be updated and change would not be persisted and thrown away. You can choose whether to use a before or after trigger.

(The design probably comes from the underlying Oracle database.)