So Cadians live on this fortress world and are trained from birth to fight and serve the Emperor. They can field strip a lasgun by the age of 5, they probably spend countless hours in PT, and they live right next to the Eye of Terror, meaning they might not freak out so much at the first sight of a daemon/chaos space marine/etc.

Catachans are rediculously over-muscled "Space Marines lite" because they live on some rediculously dangerous Death World.

Why don't these worlds (and the others I'm sure exist like them) supply Space Marine recruits?

Some worlds like them do supply space marine recruits. In no particular order…

Chogoris; Barbarus; Cthonia; Fenris; Baal; Bobt; Nocturne; Calth; Chemos; Prism; Sacris

All of the above worlds either have a strong warrior culture from birth or are death worlds with similar, if not worse conditions than catachan (sometimes both)

Cadia and Catachan already pay their tithe in the form of IG recruits. It's incredibly rare for a planet to provide both. Furthermore space marine recruiting grounds are usually gifted to then by the administratum, they can't just pick and choose where they get soldiers from.

Cail said:

I understand that Space Marines are often drawn from Death Worlds and the like. I guess what I'm getting at is why doesn't a world like Cadia provide Space Marine canidates? I mean, on a world of billions, its not going to be significant drain on resources to sacrifice a few of the preeminent guardsmen.

Whats so wrong with a planet providing both? An IG homeworld is going to pretty much be doing the same thing as a chapter homeworld, except the Chapter Homeworld will provide better training to a smaller portion of the population. I don't see why it shouldn't just be a "super-elite" training regimen.

I do understand that a lot of the out of game reason is to maintain certain themes and imagery within the setting, but in game, I see little reason.

I think it's important to remember that worlds such as Cadia are pivotal fortress worlds to the defence of the imperium. There are hundreds of warships in system any time and billions of troops in training or being ferried out to fight elsewhere. Decrees by the High Lords likely state that no other authority is allowed to interfere with the smooth operation of such systems, and that would include Astartes. Also most Astartes chapters still use worlds they were recruiting from thousands of years ago, its only the rare few like the Black Templars who are always 'crusading' and so draw from different worlds.

Chapters based on fleets probably recruit there when they passby…

Dark Templars, etc.

There is also a need of pure recruit for astartes and for Cadia the proximity of the eye of terror id quite a problem.

The world is too close it will be too dangerous to have Gene seed stock so close to the ennemy who is always willing to steal them, and it is surely quite difficult for Astartes standard to fiind people fitting their puritty requirement there.

For the vast majority of chapters (the Templars are a major exception) the home world or system the chapter draws from is important in forming the character and traditions of the chapter. The qualities that produce good space marine stock are varied but their not just martial zeal or ability. Also important to to remember in most cases chapter recruiting worlds exist in a state of carefully cultivated ignorance and tribal nature. They do not have contact with the wider imperi. Cadia and Catachan both are widely culturally contaminated by their role in the imperium.

Also as others halve said Cadia produces such good troops because it is the lynchpin of imperial defense against the eye of terra even standing up another space marine chapter pales to that. Catachan could be considered next time they have a founding but it's probably better to find a under utilized feral/death world then to repurpose one already in use.