Because it’s never that. I don’t mind developing a love interest with a companion. Or having a companion have a romance (like in Outer Worlds). But when every companion tries to get in bed with PC or with each other it becomes ridiculous. It turns from fun fantasy adventure to one of those reality shows.
And easy example of companions downgraded with romance is PoE => PoE2. We just don’t get such a varied and interesting cast in the sequel as almost everyone needs to be boinkable. I will take Sagani, Grieving Widow or Durance anytime over ripped Takehu, horny Xoti, or cool sexy Maia. And yes, I can turn down their creepy advances, but then they publically start hitting on each other, and none of them support the story as well as they could, as part of their character is designed to act as romance interest option. Honestly, I prefer Skyrim’s Sterdew Valley romances then written Bioware-like schlock.
EDIT: I must also say that I found "romancable" companions to be especially problematic in more "movie" like games aka. Mass Effect. While you can, let's say reject Miranda, it won't stop the game's camera from treating her like a hot baby that you should try to bang. I think if games were more restrained with romance, until player opts deeper into it, it would be less invasive. Especially ME3 was terrible in that regard... but then again, it was ME3.