1) they want to make sure the passengers are all at the gate ready to go if the delay turns out shorter than expected
2) they are afraid that passengers will switch to a competing airline's flight if they learn the real extent of the delay
3) the airline flight announcement staff get sadistic pleasure out of stringing the passengers along

Not one of those reasons has anything to do with the comfort, convenience or dignity of passengers.

from "the Airline Passengers Guerilla Handbook", Strategies & Tactics for Beating the Air Travel System, The Blakes Publishing Group, ISBN 0-924022-04-3, 1989. It is no longer available: that's the main reason why I pass over some of the tips I found.