Because I'm adding my own event handlers and default behaviour conflicts with my actions.
I want players to be able to navigate through the story using keyboard, space being the default "action" key. As you rightly said, this is common behaviour on many webpages. I know I can use "tab" to move focus on [[clickable]] elements, but with listener it's not even necessary to have it. Focus can be anywhere on a page, and pressing space is moving story one passage forward. I hate moving my mouse around if there is no choice to be made.
One problem I have with this approach is using, for example, "Save" option. When player clicks "Save" from the menu bar, save "popup" appears. After closing save menu, browser focus is still on "Save" button. User has to either click on passage area or press "tab" to shift focus. As result, if there is event handler active on link element within passage, when user presses "space" after closing save menu, both events fire, so "Save" popup appears and whatever function I have bined to this event runs in the background.
Maybe you can see from my description, that removing keyboard accessibility is not the only solution. Another would be to move focus from Save button to passage element each time save menu is closed.