Why the link of Hack News is not "open in the new tab"

It's annoying if a web page opens regular links in a new tab without you explicitly telling it to do so.

You could make a Greasemonkey script that changes the link targets in HN, if there isn't one already.

Use middle mouse button or cmd/ctrl+click. This is taught in Internetting 101.

That's what the middle mouse button is for.