Because people in cars (the majority) are jealous that someone else is able to do something they can't. "It just doesn't seem fair" to them.
When I rode my bicycle more I would often ride along stretches of road where I could keep pace with traffic because I knew the light timing... I'd pedal along at a steady rate while the cars launched from the light and jammed on the brakes at the next light, charge ahead and get trapped by the next light... on and on... why am I mentioning this? Because occasionally, after a few miles, those drivers would become totally irate at
me I'm guessing for "cheating" them of their right to go faster than a bicycle. That's why I always rode on the sidewalk... they would try to intentionally try to run me down, or start driving more and more aggressively until they were spinning their wheels when the lights changed or simply running the lights. The fact that a bicycle had an advantage of any sort, even a limited one like that, pissed them off.
Lane splitting does that same thing to some people. Only a tiny percentage, but they are just the ones who will act out their irrationality... more feel like you are taking unfair advantage. Of course, they wouldn't be willing to ride to get that advantage either....

Ride it like you think owning it matters.