I'm sick of people complaining about Twilight not doing royal things right away.
Cadance was turned into an Alicorn in her early teens and didn't become leader of the Crystal Empire, or do much of anything but baby sitting and travelling, until her late 20's/early 30's.
Celestia's planning on having Twilight's path take years, slowly building her up to be another Alicorn leader for Equestria, since none are needed right now, it's not like there's anything for her to do.
And being with her friends is exactly what this has always been about, keeping her with them serves the purpose of building her up.
Sheesh. Not been watching the show or something?