That's because the Tau barely have any presence in the Warp.
As a species.
Hell- when they encounter Chaos, going by Dawn of War, they have no idea what to even do.
They literally didn't know such a thing existed, Farsight is the first to find out about it and realize it's a serious issue to the Tau Empire.
1.5 imperial gallons of tea were consumed during the writing of this postI wouldn't say most of them.
There weren't that many chapters who defected that were the First Founding, if I remember rightly.
You have Sons of Horus, Emperor's Children, Death Guard, Thousand Sons, World Eaters, Word Bearers, Night Lords and the Alpha Legion, and the Iron Warriors. From Memory that's what- 10 Primarchs since the Alpha Legion had twins. And technically the Alpha Legion are supposedly double agents or something. Like- they want to do things for the Imperium. Or at least- that's what they originally wanted.
Then with the loyalists you have the Ultrasmurfs, Imperial Fists, Dark Angels had a split but I think their Primarch remained true, Space Wolves, Salamanders, Iron Hands, Raven Guard, Blood Angels, and the White Scars. So 9.
And then you have two unknown Primarchs, so there might have been an equal number of Loyalist Primarchs to Chaos ones.
And again- the Alpha Legion is debatable from the beginning.
1.5 imperial gallons of tea were consumed during the writing of this postMostly because Chaos wasn't a big thing at the start of it.
And Horus' main recruiting goal was- "Why the hell did the Emperor leave us alone on this Great Crusade?" And was essentially challenging his leadership, and thought the Imperium should be in better hands.
The Word Bearers embraced Chaos because the Emperor told them they couldn't worship him as a god. Ironically.
1.5 imperial gallons of tea were consumed during the writing of this postall i know about 40k is that we don't mention the squats
"I like girls, but now, it's about justice."

good
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UN JOUR JE SERAI DE RETOUR PRÈS DE TOI