Why does the position sticky and z-index does not work properly on table?

So i did this demo https://codepen.io/H4ds0n/pen/gOKGEVX

My goal was to take the first row and its first cell (the one with "XD" button) and put it over second row's first cell. But it seems that z-index over td are not respected. Could you explain my why?
CodePenH4ds0n
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gOKGEVX
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