Because they are sold as an innocent sexual icon that is unclaimed and "maybe you the consumer can claim them if you spend enough money on products they support".
It's this weird toxic aspect of idol culture that sees the "idol" this way whether they are a man or a woman, and whether they are an actual idol, musician, actor, voice actor, etc. If they are young and are a public face (other than politician but even then...) then odds are they are affected by this culture.
A disgustingly horrific perversion of the human spirit. We'll look back on these perverted practices as we look back on the flagellants of the 14th century, and the executives responsible should be prosecuted.
Because brand celebrities' commercial value is in the parasocial relationship that viewers have with them, allowing people to imagine romantic pairings with a person they see on TV or the internet every day while still being at a psychologically safe distance. If the celebrity has a real life boy/girlfriend, then the romantic fantasy is over and so, probably, the consumer brand attachment.
There's a class of entertainers in Japan and Korea (and to some extent other asian countries) called "Idols", for whom the appeal is being a simulated SO. The breaking of the illusion when they are revealed to have been "cheating" on you is a cardinal sin in this industry.
Mostly for boy/girl - band types, where a major draw for people is "he/she is my future husband/wife, if only we could meet once". So having a SO destroys that connection.
It's this weird toxic aspect of idol culture that sees the "idol" this way whether they are a man or a woman, and whether they are an actual idol, musician, actor, voice actor, etc. If they are young and are a public face (other than politician but even then...) then odds are they are affected by this culture.