I think it can be a pragmatic choice. If he changed after the first one but is bearable and not abusive, and you know you'll split up eventually but want more than one child, it makes sense to keep age gaps smaller, only have one other parent to deal with, and for your children to be full siblings.
The alternative is the unknown of trying to find someone new, time wasted (important if you've left having children later), issues with blended families, custody arrangements, and all of this having a massive impact on your existing child.
Plus there is the reality that there are very few men out there that actually would be significantly better than the current partner in most circumstances. Personally, I know so few men whose wives/partners aren't moaning about their lack of contribution to the daily grind that I barely believe they exist.
The COL also means many couples simply cannot afford to split up, and are making the best of things for the sake of their children's quality of life and stability. In those circumstances a second child with someone else is just not going to happen unless a random wealthy man appears and is happy to subsidise everything.
Many people want more than one child, for various reasons, and although stopping at one is getting more common there is an element of biological drive going on. And if you've already got most of the equipment, a 'good enough' situation and you can't change your circumstances I can absolutely see why it happens.
It might not be ideal, but it's some people's reality. It's not always as easy and problem free to 'LTB' as many mumsnetters seem to think, and for many it's a case of better the devil you know.