but news channels do know about the expected time as they are all ready for press conference
The meetings are big things. It not just a handful of people coming to a place and reading out a couple of pages. As it happens with every event, timelines get stretched. Even the media is given only approximate time, lets say "around 5'o clock". But that does not mean that the media reaches at 4:45 PM, has some snacks and beverages and then broadcasts the news. In most probability, the media will be arriving very much in advance and be busy setting up their uplink(camera, satellite, transmission), testing it, etc. Once the meeting finally starts, they have to wait for the numbers to be declared.