Why does itemsep not work in setbeamertemplate?

Question

I'm trying to change the spacing between itemize items in a beamer frame.

I've read that you should adjust beamer settings using the setbeamertemplate command so I'd like to follow that advice.

I can make some changes, like making the font \small and adjusting the vertical spacing before the itemize.

But wherever I try and put vspace or itemsep it seems to make no difference to the spacing between items.

Why is this, and how can I achieve, say, 5cm spacing between items using setbeamertemplate?

---

\documentclass{beamer}
% itemsep does nothing here:
\setbeamertemplate{itemize/enumerate body begin}{\vspace{1cm}\tiny\itemsep5cm}
% vspace and itemsep do nothing here:
\setbeamertemplate{itemize item}{\vspace{5cm}\itemsep5cm---}
\begin{document}
\begin{frame}{Title}{subtitle}
Foo
\begin{itemize}
\item First
\item Second
\item Third
\end{itemize}
\end{frame}
\end{document}

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