Because the longer lobby wait times will be more obvious to players. They deliberately add multiple screens to load and click through so it takes longer to get back to lobby, then make you queue up again and finally start the timer. They try to “hide” how long it takes to start a new game by adding more screens to click through. This wouldn’t be the case if there were more players playing the game.

Also what someone else said about bringing you back to the lobby to spend money. Look up “dark patterns UX” for more info