Because a summary is very useful.
Imagine you are reading about 5 years of day-in-day-out work by a team of 1500 physicists (eg. CERN, LHC, etc...).
Would you rather read the entire as-it-happened stream of research, going back 7500 person-years - or would you prefer to read a summary, in the form of a paper?
I agree that ideally the stream should be publically available as well, for those that want to consult it - but having regular summaries, written at intervals is very useful.
I'm not a scientist, obviously. And, I realize that practical science has as much to do with reputation, fame, and the climbing of academic hierarchies as it does with research. But, it would be so cool to see science more as an evolving, collaborative process.