There are few reasons for that:
1. Sprout is a temporal state and you will lose it when meet certain conditions.
2. Before you will return into NN as a Mentor, you will need to have all three roles leveled until 80 one job in each (the easiest thing), have 1000 dungeons cleared (a bit harder) and reach 1500 commendations (the most tricky part). It doesn't sound as anything serious for somebody who plays for 6 years I suppose (though I got only 1800 comms in 5.5 tbh), but for a new player who just yesterday was a sprout it's a way to go. In this period they don't have any "World" chat except /shout in every area separately.
3. At a time only 100 Mentors can be online in NN.
4. There is a thing community seem to forget: Novice Network originally isn't supposed to be a place for chatting about weather in California and that cool wedding ceremony you visited on the weekend. It's a place where people ask questions and get answers for them. But since there is no any other Server-wide chat that doesn't depend on where you are, either is an area on the map or duty, it became... what it is now. A messy can full of drama and irrelevant off-topic while actual questions often tend to stay ignored because you just fail to notice them in that scroll-the-wall mess of text.