It isn't from our perspective. However, from the business perspective of most ammunition manufacturers, in the US and Europe, it is.
Popularity dictates demand which dictates everything else, and the USA being the largest market for everything "pew pew", the .303 British is considered very niche.
The cost of tooling, logistics and set-up for a production run is quite substantive, and from what I've gathered, most cartridge manufacturers might tool up to crank out a fixed amount of .303 Brit but once a year.
That has much to do with the cost of new manufacture ammunition, and it makes sense from a business perspective, even if it offends my pocketbook as a consumer.
That's why I reload. The days cheap surplus died with Greek HXP decades ago. I keep brass to a specific firearm and handload with GC'd cast lead/antimony for punching paper. But that's just me.
Bingo, thinking I will probably keep the boxer primed surplus stuff (does anyone know if that would be corrosive?) and selling the rest. There are lots of guys that do not reload and I can sell them surplus ammo to shoot that is a lot cheaper than factory commercial ammo and use that money for components to reload non-corrosive for myself.