The main issue is it's just not realistic to create a mega server for a fantasy land based IP without a ton of instancing (phasing or overflow) of some sort. You can't fit that many players into a zone even with todays hardware. So they have to be spread out over a huge world full of hundreds of zones.
Zones take a lot of dev resources to make, even SWTOR with all the money it had didn't have the budget for that kind of effort.
Mega servers for any land based mmo (not in empty space) are only possible with extensive instancing. Not sure how TESO is doing it but you can be assured that they have put in place heavy instancing in zones that will see a lot of traffic. Starting zones, popular dungeons, cities and the like.