OK, so you won't demand that pwr_in actually be connected to a
recognizable power source. But there's a whole common class of
blunders you thus cannot detect. Furthermore, it's actually one of
the few places where DRC might catch a problem that's difficult to
spot otherwise: passive power.
As you said, the DRC has to do automated checking, don't give false
errors, and can't substitute the designer's mind. So it isn't possible
to automate it that way without giving a _lot_ of problems. It's
perfectly right if I give all the power to a circuit through a
connector, which should have passive pins. There is no tool which can
figure out where it is going to be connected there.