I don't think the numbers are balanced. For any person A there may exist a great match B, but also there may be many other A' for which the same B is a great match, but perhaps not as many other alternative B'. Even if the numbers were balanced, finding each other is a major hurdle if you're not a mainstream type. In addition, people change over time, which can affect how good of a match two people remain. So most people compromise in one way or the other, and some for which the compromise would be too big stay single.