My guess is that's because boarding a plane is a little bit like being an extra for a film, it's a hurry up and wait situation. If they printed the exact time boarding starts and people showed up then (and later), no flight would ever board on time. Better for the airline to print an earlier time and have people wait longer, so they can board as quickly as possible. Every minute behind schedule costs the airline money.

Just don't try this on Ryan Air. A good friend got stuck at the airport on a Sunday night after being denied boarding because he waited out the standing line sitting on a bench right by the gate. As soon as the last person standing walked through the checkpoint the gate crew closed the gate -- and completely ignored my friend when he showed up 10 seconds later.