Because 'end' has a specific meaning in English, and that meaning translates well to ending a block. If you decide you want a programming language geared toward Spanish speakers, 'end' would indeed be gibberish (unless they also speak English), but 'fin' would not be.

You can certainly argue that '}' is language-agnostic and therefore a better choice, and I wouldn't disagree with you. But 'end' is by no means an arbitrary choice for a character sequence that ends a block.