Why not use the performance.now () function all the time

The new performance.now () function should have better precision than Date.now (). All the sites I've read seem to only use the new function to measure runtime in search of and improve performance, because Date.now () seems hopelessly inadequate for that.

But other than what is not yet fully implemented in all browsers, is there a reason to use Date.now () again? Or have you seen another way why on earth they just don't capture Date.now ()?