From a cognitive point of view, learning a large number of idioms is more manageable for human beings than having to reinvent the mental constructs from primitives every time. This human capacity of dealing with idioms is what called "chunking" in psychology (https://en.wikipedia.org/wiki/Chunking_%28psychology%29)

Human has unlimited memory capacity and fast memory access, whereas human logical processing power is fairly limited and error prone. That's why any real human language has a large vocabulary.