People generally get horny over knights in shining armor and princesses.

Hornet is literally a princess knight. Hell she even has a white steed.

She’s a bit of a tsundere on top of it.

Hollow Knight is a very very cliche story of knights and kingdoms both the first and silksong. These sorts of stories are basically the foundation for a lot of peoples childhood fantasy stories.

So you have a very well made classical knights and kingdoms fantasy story with well done classic renditions of the standard character tropes.

Combine these two facts and you have a breeding group for people to become overly attached.

There’s a good damn reason these settings and character tropes are classics!