Because the roof will give you plenty of warning before it actually leaks through if you're paying attention to it. Then when you may really need to seal the roof, the stuff you already needlessly put on is flaking and peeling. There's nothing to do then except replace the roof. Take care of the penetrations and seams, and let the roof do it's thing for 12 to 15 years, then start considering putting a sealer on it if you still have it. There's nothing proactive putting on sealers when not needed.