Sorry but I don't agree that finding someone attractive = fancying them.
"I don't feel a spark" does mean "I don't fancy you" but it doesn't mean they do not find you attractive.
I believe humans are drawn to relationships that somehow echo those formed in childhood.
It's the reason for the stereotypical "daddy issues" some women are said to have. It's the reason lots of men go from being babied by their mothers to looking for a wife to do the same. It's the reason that children of emotionally distant parents subconsciously seek out the same in their partners and repeatedly go through patterns of trying to fix them. It's the whole reason for "he seems great on paper but..."
Those are all negative examples but its normal for that pattern to appear even in healthy relationships. Familiarity is comforting to us and when we fancy someone I think it's just our brain lighting up realising they can be something we need or want to recreate from childhood. Often it's attempt to fix a relationship or is just us reliving an aspect of it.
There's literally loads of men who are objectively very attractive and I don't fancy them, even the ones I get along well with. If i saw pictures of ten men and put them in order of attractiveness, then met them all, it's likely the ones I fancied wouldn't be the ones at the top - but they wouldn't be at the bottom either as physical attraction is still a big part of it.
I don't think it's that physical attraction + getting along = fancy someone.
I think it's physical attraction + getting along + that magic brain spark that happens for some people but not others for reasons only our subconscious is aware of = fancying someone.
If anything spark seems like the perfect word to describe what it is I experience. Evidently from this thread it isn't everyone's experience but I can assure you that it's absolutely possibly for me to find someone attractive and get along with them but not fancy them in the way I want to pursue a romantic relationship. It's not that there's something secretly wrong with them.