It's for convenience. Waiting is annoying. It's a suggestion.
But you don't need to wait. You can still move the mouse under the Steam Pop-Up in the corner. RTB's pop-up is the issue since that opens the RTB overlay.
But the "Start" button shares the same space as the "Launch Game Button." So you can click the space to the left of either overlay box. There's no inconvenience or reason to wait for it to go away.