People are stupid and greedy, but there's another reason.
Notice all those level 9 Mews trades? The thing is, Mew only comes (legit) at Level 10, so it's an impossible trade. This is because those people are cloning their pokémon. I'm not going to go into details, that's easily found on various sites, but essentially, they need to offer up a trade that can't be completed, so that while they clone, no one comes in and takes their original.
This was somewhat countered by a code that allows you to complete the trade, but it's long and difficult, and just cheating up a level 9 Mew is easier.
But yeah, mostly stupid and greedy. I don't think I'll be giving up a level 100 legendary exclusive for a level 2 Bidoof. And anyone that does... god damn you.