The biggest clue for me is look what she had - a hot, muscular, very good looking, young HUSBAND, who she left for some goofy, old Malibu sea urchin with a career in TV stage management. She's a narcissist.
She didn't leave Matty for an even hotter guy, she left that hottie for a gross, leather faced dork with a bloodshot eye.
She left Matty for herself, because she loves herself more than anyone or anything else. She thinks she is a star.
I think Spencer like most creeps, quickly assessed what she desired and seduced her with his TV experience, the celebrities he's worked with or was working with at the time. Showed off his beach front Malibu (dump) apartment. For some poorly educated narcissist, this likely impressed her, thinking he could help her get Disney vlogger famous with his *entertainment* skills.
Now she's morbidly obese, and has a growing base of anti-fans and owns no real estate, not even a car..and all of that YT revenue is going into Dencers bank account. Whenever they shop, she has to get his approval first.