These shows tend to be about groups of characters engaging in what you might call social or political struggles. Tensions, drama, conflict, emotions, and so on. Reality shows like the Kardashians are a distillation of these aspects and create something that is sweet like sugar to people who are disposed to enjoy that flavor.
I think a lot of people judge reality TV shows without watching them and kind of look down on them. If you don't have the taste for something it's easy to look down on it. For example, lots of people look down on Star Trek, which I loved when I was younger and still do. If I had to answer why I liked Star Trek, it's because I like to imagine myself as Captain Picard and figure out what I would do if I were faced with the given moral dilemma of the week. If my fantasy preferences tended towards "How would I handle the handsome jerk my sister is dating while I myself am a beautiful billionaire?" then I would watch different shows.
Madonna knew it in the 1980s. Britney or Jessica Simpson knew it in the 1990s. Paris Hilton knew it in the 2000s. The Kardashians have known it the past 15 years. Hugh Hefner knew it 70 years ago. Every model of consequence knows it.
The Kardashians are extraordinarily talented at selling sexuality. They know how to package it. They know how to market it. They intentionally replicated the system that worked so well for Kim, on to Kylie Jenner as she came of age.
Once you gain an initial position via your sexuality, leverage it into further gains. Sex rarely fails to sell.
And the parent is also correct in that it mostly (not exclusively) works for women, regardless of the downvotes attempting to change human nature and reality. A few male celebrities have benefitted tremendously from the effect, including Brad Pitt and Matthew McConaughey (how many famous male models can you name from the past 20-30 years though?).
And the target market for the Kardashians (I think, as far as I understand it) is not the people who find them sexually attractive. This is different from Marilyn Monroe, who mostly appealed to straight men, and for whom "sex sells" would be a perfect description. The Kardashian's market don't want to have sex with the Kardashians. They want to be the Kardashians (I think).