There’s a pretty consistent trend in that the newer the server is (ie. higher number), the more active the population.
Distributing new players to the older servers wouldn’t help all that much, because the ones who notice the sparse population the most are those who are long time players and are at or around the level cap. New players won’t make it there for like at least a year without spending large sums, and not enough will.
Server merges are pretty much the best way to solve inactive populations, but even those are a temporary fix before players start dropping off again.
It’s the nature of mobile games - only the dedicated players eventually remain, and that results in things like contests and PVP being much harder for low spenders - since the dedicated players are likely those who do spend.