Because she wants Sprite to grow up, not just be an adult. There's a whole process thing.
The plot actually reminds me of the cosmic side of World of Warcraft lore.
In the Warcraft universe, there are a number of cosmic forces: Light, Disorder, Death, Void, Order, and Life. Titans are planetary-scale cosmic beings of Order, and are born from planetary world souls (only a very small number of planets have these). Azeroth, the planet on which most of the game takes place, is host to one of these world souls, one that has the potential to be much more powerful than the rest of the Titan pantheon. However, when the Titans first discovered Azeroth, the planet had been infested with creatures of Void, called Old Gods, which were in the process of corrupting the nascent world soul, which would be Bad.
So the Titans' first reaction was of course "Nope", and one of them reached down to destroy one of the Old Gods. This succeeded (sort of – there were still remnants around that caused problems), but in the process the planet was gravely damaged which of course was not good for the world soul. So the Titans instead moved to plan B, which was to build a whole bunch of smaller constructs imbued with some of their power, who then went on to actually defeat the Old Gods and their minions and imprison them. These constructs were not simple automatons, but had wills and minds of their own (it wouldn't do to have the titans micro-manage them, after all).
The parallels to Eternals should be obvious. Arishem can't realistically deal with the problems of nascent Celestials himself, both because he has other things to do, and because it is hard for him to act on that level. So he builds Eternals to deal with the problem, and he makes them autonomous and sapient because they are more useful without him micromanaging them – he's got stellar nurseries to run, after all. This means they have wills and personalities of their own, which normally isn't a problem, and since he resets them after each run it usually works fine.
