Yes, I know. This property can only be defined upon creation of an attribute. Hence it cannot be changed for standard attributes - and neither for custom attributes. If you need an editable attribute, use a different attribute that is editable or define your own.
There are very good reasons why this can only be defined upon initial definition of attributes. They have to do with auditability and historical data that you keep in backups. An editable attribute just can't be the same attribute as a non-editable one.
EDIT: Non-editable attributes can only be written to by scripts. Typically governance workflows do so to record state defined during their execution or to record information to be collected only under a regime of such a workflow, e. g. in user dialogues.