Because strangely, not everyone does. And as has been observed already, it seems that most browsers do not provide a mechanism for ignoring target directives. It's fine if you want to automate it for yourself, just don't force other people.

In fact, I'd propose we implement that "set target for external links" (ie http://, google://, cpan:// etc), and have a setting for it that defaults to off - solved for everybody. (Or indeed, defaults randomly to off or on, seeing as this seems to be a 50-50 preference)

C.