People disappoint each other, critically. When they do, the massive amount of societal pressure to "make marriage work" helps outweigh the mind-bending levels of doubt, dispair, anger, and hurt that pile up on the other side.
In a society where failed marriages outnumber the successful ones, I'm not feelin' a lot of the "massive amount of societal pressure" that you're talking about.
And take a culture where there IS this massive amount of societal pressure to conform and live up to promises such as marriage: Japan. Anecdotally, marriages often wind up empty, with partners being more like roomates than anything else. Who is benefitting from the pressure to conform then?
People who don't understand what commitment is are not going to learn it by feeling pressured into staying with someone. On the contrary, I think that leads to less commitment. I would rather see no external pressure to commit, watch people fail and learn from themselves, and then learn to be committed.
Live & Learn.