According to Valve’s Idol Magal who responded to the question on Reddit, he claims that, “CS:GO will not be giftable during the sale. Our goal with sales is to grow the community and historically, during sales, the new users that stick around are mainly the ones that purchase copies for themselves.”
Basically he’s saying that if you gift someone a copy of Counter-Strike: Global Offensive, there’s a lower chance of them sticking around compared to if you were to buy it for yourself. We’re not sure if this is a valid explanation but it’s the official one that we’ve got for now. Other users have speculated that maybe this is the case to prevent third-party resellers from hoarding copies of the game only to resell them at a profit later.
There are several websites that do sell games at a lower price than what Steam sells, and in the past we’ve seen gamers run into trouble as their keys were later revoked, so maybe this is a situation Valve is trying to avoid for now.