Why are you TTC without being married. . .

I don't understand why couples who have been together for awhile don't get married before they try to conceive. Why would you make the commitment of a child without having the commitment of a spouse? It doesn't make any sense to me at all. I understand "uh ohs" and getting pregnant by accident when you at not married but if you are TTC then there is no "uh oh" it is done on purpose. Why don't you just get married especially if you have been together for a long time before you bring a baby into this world. . .