This is because most people buy on the basis of perceived value. Turning up with just one firework might make you look like a cheapskate, so why not get a box with twenty items in?
The problem is that whilst these boxes are great in some cases (like displays in smaller gardens, or for toddlers) they’re not suitable for any kind of display where you would want to make an impact, particularly for adult audiences. Here, you need good quality rockets and barrages to form the bulk of the display.
Put it like this: A single box with a dozen or two small farty fireworks is bad enough. Can you imagine half a dozen or more such boxes? Your hosts will of course have to be polite and you will have to stand and watch them all being let off. For several hours.
And to put some numbers on this, the box above (around £10) has 14 items in and an overall gunpowder content of 89g which is an average of just 6.4g per firework. A good pack of giant sparklers has around 5g!