But firework enjoyment doesn't scale linearly. Launching a single firework at random places in London is not as impressive as a bunch of fireworks at one place in London.
Large fireworks displays do scale sub linearly, just not all the way down to a single bottle rocket. The issue is large displays are viewed on a roughly flat surface aka the earth, but irradiate light into 3D space. To maintain a proper viewing angle fireworks need to be launched ever higher into the air as your showing them to more people.