It is a classic mistake by greedy people. They do not have time to care about the privacy of their keys, they want only money as soon as possible. Most of them are participating in fake airdrops: Give us your private key to verify the address and then we will send you the airdrop.. 
Ain't be appropriate to call them greedy, they actually careless for their account and lead into losing them. Not a stupid mistake cause even old Bounty hunters tend to lose their keys especially if they only save it into their laptops or PC. We cant memorize the keys in the first place and losing the keys will be losing it forever.