I don't think you understand marriage if you don't think it's an achievement.
If you give the people in question the benefit of the doubt, you would assume that they understand the gravity of the interpersonal and legal commitment they had made. Building up the trust to make that commitment is hard work.
It'd be like if somebody just made a commitment to donate periodically to charity. Presumably, those donations required that person to establish themselves financially to the extent that they can be reasonable sure that they will be able to keep their promise. So even though it's a "decision," it's closely related to an achievement.
Or maybe this illustrates it more clearly: You'd consider it an achievement to reach a 50th anniversary, correct? As you say, "many marriages end in failure." So why not a premarital relationship resulting in marriage? That doesn't always happen either. Most premarital relationships end in breaking up, not marriage.
So, as long as we're treating these people with maximum respect, we can see that:
1. Getting to the point where they were ready to commit to marriage was a challenging process. Big commitments require stable foundations, and stable foundations aren't free.
2. There is reason to hope that marriage will be a positive thing for the two who have decided to engage in it.
1 and 2 together are sufficient cause for congratulations, so they are in order.