Not everything that is hard and has a shell is also a nut.
Although the whole world talks of coconuts – coconut palms do not produce a true nut. People from times gone by were probably just reminded of nuts by the shape and hard shell. Pragmatic, you could say. It was all scientifically sorted out a long time ago. Coconut palms produce “one-seeded drupes”. But you can continue to say coconut. It sounds much better.
Nuts that contain juice, that would be quite strange really …