Why are the keys on a keyboard square?

Some of the keys on a keyboard are rectangular in shape, and the Enter key is often a strange shape, but the majority of keys are square. But there could be other possible shapes for those keys.

For example hexagonal keys could fit well with the staggered layout of keys, and reduce the distance between adjacent keys in a diagonal direction.

(This could be skewed to provide the same positions we are used to.)

I see some manufacturers have also created some keyboards with round keys, but I have never seen these in use, except on very old typewriters.

So is there are good reason why keys on a keyboard are square, and why a hexagonal layout has never caught on?