For the opportunity :)
And welcome.

To be nice and all. Some people do blacklist for showing gratitude, but they're no fun at parties.

Could be lots of reasons, some feel it's common courtesy, a lot of groups used to have a comment or get kicked rule and it's just automatic for them, some are bots, some just like filling up your unread messages thing.
Also, welcome to the site!

it's because they are giving away a game instead of keeping it, so we thank them for being charitable :) and because theres a chance to win. Thanks btw! :p