Because the Earth isn’t just one solid lump of material. Instead it’s made up of layers. The inner and outer cores, where almost all metals are actually located because they are dense and so sank into the core eons ago. You then have the mantle which is rather runny and so moves around. You then lastly have what you’re used to which is a thin layer or solid crust. So there’s no way for it to be equally distributed.
As to why veins? Basically it’s where there’s a gap in the crust and it gets filled from below.
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