For example, Flash Gitz represent the pinnacle of Shoota technology. They understand it as much as anyone ever will, and they won't pass on their knowledge to the next generation because the next generation will simply grow up to be Flash Gitz. In 80k, the Orks will not have bigger and flashier guns, except by looting Tau weapons.
Also, it is possible to avoid warp possession through faith and willpower, and the Ork's faith in Gork and Mork is unshakeable.
It's also worth noting that the Eldar only have to be super careful because there's a chaos god waiting specifically for them, who has a unique bond with them. Pre-Slaanesh, they were a race of latent psykers that didn't live in constant fear of daemonic possession.