The Biblical word “covet” relates to this phenomenon. I also agree with the previous poster about the situation in which a man reaps the benefits side of a relationship without the cost of the responsibilities side - that may be attractive.
Finally, I would offer the simplest explanation: a married woman is still a woman, and as such, can be the most attractive and desirable choice for a single man; not everybody would deny himself the pleasure of seeing the woman who happens to be maximally desirable for him just because she is married already.
In essence, it is like choosing a business with positive Yelp reviews over a business that has not garnered any reviews.
As the Russian literary canon would explain, dating an upper class married nobility woman was considered a rite of passage single nobility men strove for - review the plot of “Anna Karenina”by Leo Tolstoy for the most famous example.
Some marriages can recover from an affair, but many will end in divorce.
If you are getting involved with a married woman, you should spend some time seriously considering whether you are willing to break up someone else's family. In many cases, a married woman is playing the field simply to find a physical relationship.
However, the married woman you date may fall in love with you or decide to end her marriage to be with you.