Help a non-American understand the Border Wall argument that Mexico should fund it

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I’m not from Canada/US/Mexico, so have little understanding of the politics & culture issues at play.

But if you want a new fence, usually you split the cost fairly with your neighbours.

So I don’t understand why conservatives/Republicans argue that the US doesn’t pay, and are putting all the financial expectations/responsibility on Mexico.

Can someone explain as simply as possible? I’m genuinely interested in what’s going on.