In my code, I've write defalut_get method and it is perfectly working when I create new record. but, when I open existing record default value is not showing

here is my code:

@api.model
def default_get(self, fields):
    print('fields ------------', fields)
    res = super(SalesTarget, self).default_get(fields)
    activity_stage_target_ids = [(5, 0, 0)]
    stage_rec = self.env['crm.stage'].search([])
    for stage in stage_rec:
        line = (0, 0, {
            'stage_id': stage.id
        })
        activity_stage_target_ids.append(line)
    res.update({
        'activity_stage_target_ids': activity_stage_target_ids,
    })
    return res