Shooting technical expert
There are two main reasons. The first is that, for many years, cartridges in the UK have been much cheaper than in the USA, and most other European countries.
Price comparisons now show not that much difference between US and UK prices.
Nevertheless, British shooters never really acquired the reloading habit. More importantly, perhaps, you may not use home-loaded cartridges in CPSA competitions.
There’s the safety angle to be considered, plus the possibility of loading more than 28g of shot in a cartridge, and therefore cheating.
I don’t think many shooting grounds would allow you to use home loads in informal or club competitions, either.
You could load your own for field shooting, but you would have to load a very large number of cartridges to justify the cost of the equipment and components – unless you get together with a group of friends to share the start up costs.
As you suggest, the only type of shooting in which home loading makes any sense is wildfowling, and then only if you shoot a big-bore gun for which the ammunition may be either very expensive, or unobtainable.