Volume. In a word. By default, the hub and spoke system looks, on the face of it, to be complete madness...
Actually it is the work of genius as it enables all items being delivered to any given single locale to first be gathered at a single central point...
The system does have one slight flaw... the various hubs and depots need to ideally be located no more than 4.5 hours apart for drivers hours reasons... that's why anything going to an island or north of Edinburgh/Glasgow cannot be guaranteed to arrive any sooner than 48hrs...
If you need it there sooner, either as a customer or supplier, then you're looking at a dedicated vehicle and suddenly your £5 shipping becomes hundreds.
But if you can get a thousand items all paying £5 to use your system, then you have £5,000... multiply that by four trailers from 40 'spokes' then you have £800,000 a day, six days a week, 52 weeks a year and you have yourself a £249.6 million turnover...
40 depots might have a combined fleet of 1,200 delivery trucks and 160 artic units and 400 trailers... and perhaps 20,000 employees...
The numbers are fictional, as some will pay less and some will pay considerably more to ship any given item of any given size or weight, but it gives you an idea of scale.
Don't forget, this is purely an illustration of one company and is purely the shipping costs not the value of what's actually being shipped.