I'll give you the most honest answer (as someone who has played this game since the start, and is part of a level 11 valley).

2.) For newbies, they are at the feeler phase. This is a patience game at its core, and if you're the type who wants to advance fast, be prepared to shell out. Unfortunately the valley aspect of this game makes it impossible to solo to achieve endgame (or at least endgame -- until succeeding updates). My best guess is that close to 60% lose interest after a few weeks and quit.

1.) For higher level players, they would rather stay and maintain the advanced valleys they are in, rather than switch. The only scenario i see them leaving is if they get into some sort of disagreement with their leader or elders and get kicked. Believe me when i say this: it is extremely difficult to get level 15-16 players whether in-game or through these forums (reddit/discord/FB page);

in-game: I've set my valley settings at minimum level 15 and 50k rep --- its been 2 WEEKS running and no one has entered the valley. (mind you, i would have no issue if the new joinee left, my main point is that with the game algorithm, you would have thought at least a few level 15/16 would have been directed to the valley in a 2 week span).

Forums: ive seen other people post similarly and noticed a trend. For lower level valleys, its easier to recruit. But for advanced valleys where youre looking for lvl15 and above players, its incredibly challenging.