If you've ever looked at your world's player list, it's clear that there are many players that play without benefit of guild membership. Do you suggest that those players would no longer be allowed to play unless they joined a guild?
Require players to join a guild?? Whats wrong with letting them choose whether they want a guild or not? Think also of the poor quality of guild support one is likely to receive from players forced into joining a guild....Thats a bad idea all around.
There is an organic progression for new players. The guided quests that introduce buildings, roads, supplies, coins, etc. take the new player into the Bronze Age. As play progresses, chat becomes more useful. And, some guilds actually mine the bottom of the standings list for new members, both as a continuous influx of new blood, and as a service to the game.
So guilds come into the new player's view at some point, hopefully about the time they can make an informed choice.
The random and organic manner of play is more likely to meet more needs than a requirement to join a guild would meet.
And the size of the guild is large now, when time to m/p and assist GB growth is counted, let alone for the guild leaders to manage.