Ever try blowing air into a bottle?
Can’t do it!….unless you put a hole in the bottom of the bottle.
Venting a room is much the same. It’s only successful when the room has a means of allowing a volume of air to escape (being displaced) so as to allow fresh or cool air to enter.
If you want the cool air outside to enter the room you need to provide a means of displacement.
The only means of efficient displacement is up through the ceiling into the roof void.
Obviously having the volume escape out of the roof via an efficient roof ventilator is also beneficial.