Many (if not most) of those "blackpilled" incels will answer about this that it's because of the beta-buxxing (i. e. the man is ugly but he's rich so the woman is with him because of the money, property, etc.) There really are cases like this and nobody can't say that there aren't women who are just following the capital. But it's only one part of the cases. I don't know how big is this part but it can't be 99% or even 80% of the cases. Let's assume that 60% of these couples (ugly man with pretty woman) are a result of the concrete females' greed, pragmatism, utilitarianism... but there are still 40% of the couples (or what seems to be a couple) in which the ugly men aren't rich, aren't from noble families, haven't powerful relationships and so on; they're just average. So why they have pretty women? Possible reasons:
1. There those men who in the eyes of many other men aren't handsome but for some of the females they are handsome. For example, all those feminine males (like some of those J-pop, K-pop, etc. bands). Many men will think that these ones are not attractive because they do not look like some Chads like John Cena, The Rock, George Clooney... they're even not looking like some Prettyboys like Francisco Lachowski but there are females who like them.
Also, there are these cases, when a man can think that another man isn't more handsome than him but in fact he has better or/and more attractive traits. For example you're thinking "Look at him, he's shorter than me and he has no muscles! I am better than him!" but in fact she thinks that his voice is better than your voice, his character (personality) is better than yours, his smile and gestures are more attractive than yours, his skills are more useful than yours, etc.
2. There are those women who are tired of being with handsome men (had issues with this kind of men, like many other women competed with her for the handsome boyfriend; the Chad cheated, etc.) so they on purpose choose an average or even a bit ugly man; in this kind of relationship they feel more secured.
3. In some cases the man was much better looking when they start to date. Later he lost muscle mass, gained weight, get older... and now he looks worse. But she is still with him because she used to and/or she loves him.
4. In some cases he is not a boyfriend, lover or husband. He is just an orbiter, a friend, a brother, a colleague... or even an ex-boyfriend.
5. Sometimes these couples are a product of long, hard warm approach (term that described romantically approaching someone that one is already acquainted with instead of approaching a stranger). Sometimes the persistence is working (and for some females it's even a must).
Not always. Sometimes it's counterproductive. 6. It's just their first (and probably -- last) date.
If you can think of more reasons, you may share them here.