Because there arent only brand-new sprouts. There are also level 70 sprouts - dont think of sprouts as only the people entering Sastasha just now. Thanks to jump potions and the like a sprout can legit be level 63.
I'd also see it as some sort of advertisment: Showing the sprouts "Hey, there is more cool content ahead of you!" potentially leading to questions and explantions about the giant-fate-mobs in general (Odin and Behemoth could be kinda relevant for example) and the hunts in general. Thats at least how I would do it... besides the whole breathing some life into the channel thing - one of the more important points to the NN in my book.