If you intermingle finances or kids or property ownership then you have all the problems of a divorce except no legal framework to handle them in. Which makes for an even worse splitting process rather than a better one.
Right, you potentially miss out in cases that can really matter (e.g. power of attorney, things related to children) and you might find you fail to avoid the things you’re trying to avoid (e.g. governments/courts giving some recognition to long-term relationships)