There is a good reason it is not available. Marriage is a couple fact, as in it takes two people. When you are adding a person, the spouse may or may not already be in the database. If they aren’t, then there isn’t a second person to attached to the fact. If they are in the database already, there is not facility to select the second person.
So you have to add two individuals and then add the marriage so that when it gets written to the database, it will be attached to both people.
You can sort of change the order a little bit if it makes your data entry any easier. After having added the first spouse without a Marriage fact, you can edit the first spouse without having first added the second spouse and go ahead and add a Marriage fact. You will be prompted at that point to add the second spouse. Sometimes (but not always!) I find that to be a smoother workflow than adding both spouses before adding the marriage fact.