Why are there an uneven of servers in each data center?

[GMERC]

And why can't people in the same data center play with anyone outside of it?
Is this a design choice?

Yes, that is the entire reason for Data Centers existing.

Would you prefer balmung or gilgamesh moving to primal?

All a stupid move like that would do is move the "server load" to Primal Datacenter, only it would be worse than anything Aether currently experiences.