Most Helpful Guy
Many possible reasons (fear of being alone, skeptical of a future or better relationship, trying to work things, etc.) and I hope such relationships aren't common.
Incompatibility and financial problems are probably the only relationship killers, the latter being curable and the other not lol.2
Most Helpful Girl
Social psychology 101: the more we invest, the less likely we are to walk away. Same reason people lose it all in gambling, stocks, blah, blah, blah.
We invest, because we think we've chosen a good investment. If it goes wrong, not only do we have to cut our losses, but it also tends to shine a mirror back on ourselves for "having chosen poorly."
Nobody likes to think they picked a bad investment and are now having to deal with loss and fallout because of what they chose. So... people stay, they blame the other, they rage at all possible external forces to make sense of/alleviate the guilty and responsibility of "their choice" going south. It usually takes something so aberrant that the person is willing to accept the losses rather than continue. Examples: people tolerate shitty attitude, but don't tolerate cheating.1