Because Warhammer, at its core, is a product of British liberals. You might as well ask why the post-scarcity utopia the Aeldari had built devolved into a nightmare of bloodied hedonism so terrible, it literally gave birth to a god of excess.
If you mean lore wise, I am pretty sure it wasn't always that way (before the Imperium, that is), but a looong chain of bad decisions and various fuckery made it inevitable
Btw abour Aeldari, it's very likely that Emperor sabotaged them.
Their civilization lasted for 60 million years which means incredible stability not known to any other species including Necrons and possibly even the Old Ones. But they fell only in last 10-15k, which even for their personal lifespans seem very abrupt. There is also confirmed in lore there was no outright hostility between humans and Eldar (they allowed humans to spread and traded) before their fall, which means that decadence was grossly overstated - their entire civililization just couldn't be at the level of of current Drukhari as it is claimed many times (iirc the source of those claims seems to be Asdrubael Vect who is worst possible source on this).
It's basically confirmed that Emperor greatly feared the Eldar since he might be most powerful psyker in the galaxy, but Eldar had probably billions of beta class psykers and above. Also he only came out of the closet after Eldar empire was truly confirmed destroyed, the exact date is incredibly suspicious. Most of canon books place the fall of Eldar as a process that ended or even happened up to literal begining of the reunification of Earth, which means Emps waited exactly as long as it was necessary for Eldar to stop being a danger to him and then abruptly moved in a blitz.
All that is way too coincidental, it's also very fitting with all the other cases where Emperor actions ended up empowering chaos.
T'au are basically Brits screaming Orwell Animal Farm 1984 so hard that they accidentally made the T'au based.
Literally every sentence about them though is trying really hard to imply something evil behind it, or somehow implying that Ethereals enforce their philosophy of equality and respect with Jedi mindtricks.
The 9e codex went really hard in on the T'au doing controlled breeding for eugenic purposes, but never explained how it actually worked. Of course, GW being Brits, they've kinda done this for just about every faction at this point so it's hard to argue that it makes the T'au particularly evil in the setting.