Would it seal better? Of course.
However, the o-ring would add drag to the bolt, countering the efficiency gains.
Also, there isn't enough material in the bolt in that area to add an o-ring groove. It would require a new bolt design. You would have to decrease the flow path of the gas to gain enough space for an o-ring. That would also lower efficiency.
Additionally, the gas when leaving the bolt is being directed forwards. It has to turn 180° to escape between the bolt and barrel. So not much is escaping via this path.
Plenty of other markers lack an o-ring on the bolt tip. It isn't a critical component.