I'm not sure if you've said anything there that contradicts what I'm saying. I would say that a YouTuber can reply to the entry comment (shows up in your e-mail inbox for the majority of people and for the rest it'll show at the top right of YouTube as a glowing bell thingy).
I get why giveaways are done, but you're forgetting that in themselves they provide publicity without the need for sharing/liking/subscribing: They're a way of reaching a new audience who may not even watch your videos before that, but that doesn't necessarily mean you have to ask them for likes and shares etc. I 'unno, just seems slightly cheap to me to make content that relies on someone having to do a bit of a song-and-dance to get engaged with the content creator and almost acknowledges the fact that ultimately your views aren't enough: It needs to satisfy the sponsorship agreement they have with whoever is sponsoring the giveaway.
Perhaps it is a naive thing to think, but I subscribe (and share) to content that I like to watch. Just feels like very lazy content creation to me with a lot of the recent airsoft giveaways. The thing is, I'd happily share most of these videos anyway, but don't like the idea of being forced to very slightly dilute my timeline and subscription tab just for the sake of one YouTuber's mortgage - not even if I'm only doing it for a week before unsubscribing again.