I'd say a lot of it has to do with him being the only person who is willing to pay attention to her. There aren't too many in real life options for a crazy idiot covered in horrible tattoos who lives with her parents and has no real personality. And I think she's probably too paranoid (and rightfully so tbh) to try and find a guy online. It would almost certainly end up being some legbeard from FB trying to troll her.
But she also has BPD, and usually people with that decide one or two people are like, the light of their lives. You can see it with Luna Slater for example, who has wrecked her life to be with a dope dealing old man who used to bang her mom. Hogg is probably that person for Abby. The bigger question to me is why Hogg puts up with all the drama that comes with dating Abby. He doesn't really get anything out of it.