Exactly. NO one was thinking at that time the car was a crime scene.
LE had access to the car all along, but they were initially focused on Casey's story and trying to find a kidnapped and hopefully alive Caylee. They investigated the car the next day, after the lies were confirmed and someone mentioned the smell again.
The tow yard guy who'd smelled death before wasn't thinking as he chucked the bag over the fence that maybe it was evidence and he should leave that bag in the trunk. He may have thought about it later, but at that moment he did not.
Hindsight is 20/20...
Everyone jokes about it, but I think when Cindy learned from George about the stinky bag of "rotten pizza" in the trunk, she truly believed in her own mind that is what caused the smell. Unrealistic, but given the state she was in, it had to be the answer.