Why border-spacing not work with border collapse property

Question

https://code.sololearn.com/WT86T1FZZORe/?ref=app

Answer 1

border-spacing works when the border-collapse property is set to "separate".

Literally, "border-collapse: collapse" sets the border-spacing property to zero,

Answer 2

border-collapse: collapse; do much more than setting border-spacing to zero... it merge cells borders as one, but border-spacing: 0; makes borders be adjancent ^^

so border-spacing has no meaning when using border-collapse: collapse ;P