Where are those giveaways coming from?

Google does give personalized and regional results, but it was the first result for me, with their Twitter being third. Givee Select, the payment service, was second. Perhaps "givee club" would give better results.

From the SG guidelines:

The below is a list of content that is not allowed in our community. Content consists of any material you post or make available through our site, such as discussions, links, comments, usernames, or Steam avatars.
...
16. Third party giveaways that require or reward users for performing profitable actions. For example, linking to a giveaway on another service that requires or incentivizes users to click a referral link, like a Facebook page, follow a Twitter account, join a Steam group, complete a survey, sign-up for a newsletter, or make a donation.

Humble, Fanatical, Itch.io, GOG don't meet those restrictions so it's fine linking them. Most giveaway sites do require performing such actions, so that's why we can't direct link them.