It's like 3 or so lines in userscript if you really need it.
In general case most modern browsers allow an alternate way of opening a tab in background, e.g. middle mouse button click, so this gives users a method of control over what they want hyperlinks to do, but not the other way as there's no easy way to undo target=_blank.
It's usually considered bad UX practice to do so by default unless clicking a hyperlink can cause irreversible state change like in some cases of autopaging.
I semi-appologise for the sigh and the snark and I'll take the downvotes as being merited for my tone but the number of times I see posts like this (aka "Why Doesn't HN do xyz?") I cry a little inside. There is a search box at the bottom of each page (which is what I used to provide the links above rather than some super-human feat of search skills) and that search box should be the first port of call for any user that feels the need to post a 'why does HN do / not do...' i.m.o.
In general case most modern browsers allow an alternate way of opening a tab in background, e.g. middle mouse button click, so this gives users a method of control over what they want hyperlinks to do, but not the other way as there's no easy way to undo target=_blank.
It's usually considered bad UX practice to do so by default unless clicking a hyperlink can cause irreversible state change like in some cases of autopaging.