From my own experience, staying in contact or still seeing an ex after a breakup is a way to delay confronting the pain of the actual breakup.

The dumper has a mindset of, they still want to spend time with me, they must still have feelings for me, maybe eventually we can get back together. While the dumper might regret the breakup and have either a similar mindset or is feeling guilty and its a way to feel less guilty.

Either way eventually someone's going to get hurt. I carried on being friends with my ex after a 9 year relationship because I had to stay at hers to see my daughter as I work 3 hours away. But that led to us blurring the line between friends and a couple quite a lot. Like we spent days out as a family, went for meals out, went on holiday, planned holidays together a year in advance, bought extravagant gifts for each other. And in the end it became too much, I realised I was never going to be able to move on if I was doing all that so it was a case of we either go on a date and see where it goes or we end the friendship. And she chose not to go on the date and end the friendship.