Because the average HN user is perfectly able to middle-click or ctrl-click or long-tap (or whatever combination works in your specific environment) when he wants something to open in a tab? It's not the AOL target audience.
It's a lot easier for a user to force a link to open in a new tab than it is for the user to force the link to open in the same tab.
Because it is better to have the link open where the user specifies. This is why browsers should control where links open.