Because birds are among some of the most intelligent land animal (some bird species more than others).
I don't know about this. If we look at their behaviour in the wild, I think this says it all. They spend the majority of their lives in a burrow - not seeking enrichment. Perhaps when we take them out of their natural habitats (even if they're captive bred - instinct is instinct) they tend to stray around their tanks more. I don't know, but you cannot compare birds to snakes in terms of intelligence.