Why is 'in danger' considered by most conventional dictionaries to be an idiom?

Merriam Webster defines an idiom as:

an expression that cannot be understood from the meanings of its separate words but that has a separate meaning of its own<

It then goes on to categorise the phrase ‘in danger’ as an idiom. It literally means that someone or something is facing a danger of some sort.

I would imagine an idiom to be something more like ‘a piece of cake’ or ‘spill the beans’.