Interested more out of curiosity than anything. I don't understand this idea of cohabiting, birthing kids together and the sharing the responsibilities of parenthood without a solid commitment like marriage, or at least engagement?

Take for example, Michael Shannon, one of my favorite character actors. He started dating his girlfriend in 2002, had 2 daughters, and only got married 3-4 years ago. That's nearly 2 decades of raising children and cohabiting, without any serious commitment. Obviously, celebrities are in their own class, but I don't understand:

If a man is really interested, shouldn't he want to solidify a commitment? Otherwise, isn't he just holding out on the off chance that something better comes along?

Moreover, why do women tolerate this?