You can't sneak a legal arrangement on a couple by default. If a couple want to be recognised in law, with all the protections and privileges that engenders, they need to go and make an active declaration with witnesses. I.e. get married. A contract is then formed which can be removed if the couple desire.
It's not something that can, or should, happen passively. Otherwise how would you define it?
Is it living together? And for how long? I've lived with some of my housemates for much longer than some people have been with their partners and I'm definitely not in a relationship with them.
And what happens if you want to live with someone but you both have children from previous relationships and you want to keep your lives and assets legally separate? Will the law say "no, tough, you've been living together for X years so you're as good as married"?
Is it if you have children? I think the number of single parents shows pretty clearly that's no proof of a solid commitment.
Perhaps if you have children and you've lived together for more than X years? But then that would discriminate against those who don't or can't have children. And some relationships move faster than others. Will there be cries of outrage from those couples whose relationship has resulted in a pregnancy after only 3 months that they don't qualify should they break up one year later?
And how would it end, if you wanted the relationship to end? There's no contract to annul or cancel. Would you really want a bailiff chasing your de-facto "husband's" debts from you, two years after he moved out? How about if he tried to make a claim on a house you bought after the break up? Because after all, you lived together for 15 years and have three children, so you're as good as married and the assets are joint.
How would it work if one partner died and two of their ex long term relationships came out of the woodwork to try and claim the inheritance from their "spouse"? What if you'd been with one for 25 years but had been with another for 4? What would happen if they both tried to claim the bereavement allowance?
Do you see why this cannot work?
If you want to have your relationship recognised in law just go and sign that piece of paper.