Yeah there is no valid reason not to be in a FS.
Even if you can't find one with rules that are casual enough for you, you should make a FS of your own and invite no one if you wish.
The main reason is tournaments.
Relics are important.
A normal player will get between 70-99% of their relics from the tournament.
You must be in a FS to enter the tournament.
The players you see without a FS are either mobile only (so they can't see messages) or abandoned cities. There may be a sprinkling of players who dont know about tournaments too.
One of the biggest reasons why it is so hard to fill a FS is because players are super loyal, and would rather play with 16 friends than switch to a FS that better suits their activity level and goals.
I'm not saying that's a bad thing, just an observation.
That, and the fact that making a FS has no cost, limits, or requirements. So we have hundreds of partially filled FS and only handfuls of full ones.
"Too many chiefs, not enough Indians"