Because we’re honest about what Sheldon is, and don’t pretend. He’s the kid who develops a hopeless infatuation on the first girl who treated him nice and follows her around like a lost puppy, hoping to win her heart like some sentimental sap. To her it was probably just an act of pity or empthy. She’s not interested in him that way whatsoever.
But that doesn’t stop him, because this sap has consumed a steady diet hopeful parables and saccharine stories, and has somehow developed the naive notion that if he tries really hard he can achieve anything. So he tries to win her over the only ways he knows how, except he’s a socially maladapted little sap and she’s way out of his league, and that try as he might, he’s not cool, or handsome, or charasmatic enough to pull it off.
Everyone who sees knows it, everyone can see it, everyone except him, who lacks the abilty to understand social cues and see the writing on the wall, and is still driven by this toxic notion of hope, and nothing short of an explosive reaction will ever change that. But no matter how annoying, or embarrassing, or uncomfortable he is, nobody wants to be the one to kick this pitiful creature that’s done no harm and potentially knock him back down over the edge he’s carefully teetering on. So they leave it alone, hoping he’ll get over it, or at least develop the discretion to bury it deep inside, and that it won’t come to that end. It’s like watching a goat try and break a brick wall by ramming into it repeatedly. Everytike he does he gets more bloodied and battered and You just want to stop it, but you can’t interfere for fear of making it worse. All you can do is hope he’ll wisen up and quit before he’s no longer able to.
That is what Sheldon is. An exercise in Futility. Everyone here has known one, hell some here might have even BEEN one. And i can garuntee, that no matter what idealized image you have, you’d do the same.