What an interesting question!
Hmmm, I think its due to degrees of empathy?
So my daily life is like nice, ok , good, chill. When I read manga//webtoons/watch tv/film/theatre i'm able to experience so many emotions that I wouldn't on a regular day, binging a series got me like
etc and that experience, that excitement, that entertainment is what gets me addicted to a fictional story
Geniunely feeling physiological reactions to the pixels on my screen, ink on the pages, like butterflies in the stomach, tears in my eyes, goosebumps etc that is what I seek out for fiction
Now different emotions have different 'thresholds' to unlock them in me. For webtoon format, fear is by far the hardest/highest. I can hardly find a webtoon scary, that's super rare, whilst the easiest to unlock is, you guessed it, excitement from romance~
That doki doki feeling the serotonin/dope just flows easily from a scene with my faves flirting
but the serotonin from jump scares is harder to get in this specific medium. I attribute it to different levels of suspension of disbelief and easier for me to empathise with an awkward confession than being chased by serial killers lol
So ultimately I'm seeking more stories to give me that happy hormone, I end up reading a whole lot of romance
That being said, only a sixth of my all time favourite manga, one twelfth of all time favourite movies, one quarter of fave anime, and one eight of my fave tv shows are romance despite being majority of my consumption, so romance obvs doesn't make a story better to me, just increases my likelihood of reading it