Most of the people who do believe the bones were planted (and there are some who do) believe the devil planted them to test our faith. Others believe 6000 years ago when we were created Adam and Eve and their decedents lived with the large dinosaurs, and recently opened a $27 million "museum" to prove it.
Creation Museum - Creation Museum
Now with all the cave paintings we've found of people painting the creatures they've encountered (Wolly Mammoth, deer, antelope) you'd think one of them would've painted a huge fucking T-rex or brontosaurus running around if they ever saw it.
Jesus probably would've seen better uses for all that money too.