YANBU, OP, and I think this thread bears it out. Lots of people on it are happy to point to their own extended families or their friends' as examples of parents with clear favourites, but everyone says 'Oh, no, I don't have a favourite child myself, that would be awful.' So it it only ever other parents who have favourites?
I do think it's sometimes a lot more complicated than certain children needing more attentive parenting (thereby looking like they get the lion's share of the attention) or simply being personalities a parent finds more compatible, though.
In the two most obvious cases I can think of, there were other reasons.
In one, the mother almost certainly had bad, undiagnosed PND (this was Ireland in the 70s, where such things were routinely under diagnosed), didn't bond at all with her third child (of six) who (cause or effect?) was a whiny, puny, difficult baby and small child, and whom she spoiled outrageously later out of guilt. He is now a rather tormented adult, though it would be glib to say there's a correlation.
The other case involved a family who were very upwardly-mobile, off the back of the father, a working-class self-starter, who was desperate to distance himself from his background and 'married up'. I was friends with his eldest daughter when they were still living in a small rented house in a grubby area and she went to my parish school. Within a few years, my friend was at an expensive private girls' school with a pony, skiing holidays and elocution lessons, and they had bought a beautiful Regency house with land where the father used to entertain business clients. I saw her very seldom after that.
However, the father couldn't quite eradicate what he saw as a 'working-class taint' from his eldest, a grubby tomboy who liked doing cartwheels in torn track suits, and made no secret of his preference for his youngest daughter, who was born after the change in their fortunes, and who grew up acceptably middle-class.