Because in everyday life we do not formally define ideas.

Just for example I play a game with my children where we tickle each other when we see a yellow vehicle. Is a yellow tractor a vehicle for purpose of the game? How about a yellow baby cart. How about a billboard with a photograph of a yellow baby's toy car?

We honestly have more fun discussing what is a vehicle for purpose of the game than we do tickling. Especially when it becomes clear that a yellow baby carriage is a vehicle only if it is daddy to be tickled... Honestly a good lesson to learn early that life is not fair and that rules are subjective.