There is more to installing a headset than just getting the cups into the ends of the head tube.

The whole idea here is to use an assy. method that will assure the two bearing races will end up parallel and concentric within a very close tolerance. This is why reaming and facing is recommended.

If the prep is done properly the cups can be tapped into place (I am not recommending this, just acknowledging that it has been done successfully) or pushed in if the cups are cooled sufficiently.

The only way to assure that the races are positioned perfectly is to press them in using a tool that holds them concentric and parallel position as they are inserted into the head tube.

I know that even in many LBS these expedited methods are used with some degree of success but we all also know that there are many bikes out there that are constantly needing headset adjustments and tightening which is a symptom of poor headset seating.