>>31913330Short answer: Google retardation
Long answer: Back when google wanted google+ to be a thing, you might remember that your youtube channel automatically got a google+ page and you were asked what name you wanted to use for it, the options being your actual name or your nickname/channel name. When you chose nickname, your channel became a "brand account" since (unbeknownst to you) you just tied your channel to a name instead of an individual. So even after google+ died, your youtube channel is still a brand account and will remain that way until you go out of your way to change that which is a whole involved process. But the relevance to you is that brand accounts cannot be gifted memberships.