What outlasts a vow is whatever object you leave standing where it can accuse you later.
Resolve fades at the rate the state that produced it fades, and the belief goes with it. So the commitment gets bound to something that outlives the mood. A stone set where everyone walks past makes denial expensive later, and avoided pain pulls harder on behavior than remembered intention ever does.
No one walks away from that ground holding only agreement. The marker cues the state, the state holds the conviction that Yahweh's words were heard and answered, and that conviction decides what gets taught to children who were never standing there. Without it, a people who meant every word keeps the feeling a season and loses the covenant by simply forgetting they gave it.