I think they are doing it to avoid confusion.

From reading the post on the lodestone they will rename servers 1-10 after the original servers.

so after the dust is settled we'll have servers named:

Cornelia
Kashuan
Gysahl
Mysidia
ect ect ect.

But that doesn't mean the current Cornelia will be the new Cornelia.

I'm sure there will be some players out there who will look at the server merge list and say

"Oh I'm already on Mysidia, I'll just stay here instead of move to a new server"

come merge day they didn't pick a new server, so SE picked a server for them.

You know this above scenario will happen.

So everyone gets to pick a new home. Only to be given an offical name after everythng is locked down to avoid more unneeded confusion.

just my 2 cents.