There is no answer to this. It's just accepted as an ATC dogma. 😂😂 Applying the separation minima at all times would be too impractical.
The minute the inbound goes around and the departure has just taken off, you have a loss of separation. If they land, you don't.
No one knows why. It's the way it's always been and shall remain enshrined in mythology. 😂😂