You're right that some data centers used to have more worlds than they currently do. For example, JP had more than 8 worlds on each data center and it caused massive issues that NA and EU did not have.
You can think of data centers as, literally a book case full of computer towers (the technical term is server racks). Then imagine that each book in the row is a separate computer tower hosting several zones (ie. Azim Steppe, Sea of Clouds, 6 wards), while each row is a world.
https://www.pcinvasion.com/wp-conten...ata-center.jpg
Adding more worlds to the same data center is possible, but it seems the overall amount of network or database traffic from that would cause problems. Although I do not fully know their server setup and issues, each data center seems to have a different IP address and thus I assume a different wire to receive traffic. The heavy duty wires they will use will likely handle a lot, but at a certain point it's going to reach its limits and so are the server machines themselves. And let's not forget people test that regularly with very effective DDoS attacks.
Thus, to split the traffic and support the growing population of the game, they had to create separate DCs. Older players will remember Aether and Primal splitting apart into Crystal and there was probably a similar story for Chaos' split into Light. There was no DC travel then so friends were lost, FCs broke apart and people quit. JPs also split although I think these happened closer to the time we got DC travel.
Most likely some games handle this using cloud servers or otherwise being less strict about it. They chose against cloud servers because they are technically slower, but in the cloud test they performed (which I participated in), it was fine. Cloud servers are basically less strict. They can move around location and increase/decrease hardware requirements through the power of virtualization to stretch over the differing requirements needed on a given day and potentially split up the traffic depending on the person's region (this probably isn't good for a game that depends on fast updates from a central server but can work for websites and thus help with DDoS attacks).
Although I think making a mega DC would go counter to all the work they have done splitting them up, I have often thought they could try to break down the barriers twixt data centers in specific cases - for example, putting duty servers and housing servers each in a separate data center and coding the game in a way that switches data center to the region's duty/housing data center, without logging out.I was thinking about it and honestly I think if they found a way to merge DCs or make a mega DC they won't have to think of a cross DC PF?
This would require them to rework their whole data center instructure setup, and add a lot of code to make it as seamless as possible for the player. I wouldn't rule out that it's something they do in the future because I used to think "they could do world visiting and DC travel but they won't", and they surprised me by actually doing it. These were things I thought MMO developers were too old fashioned to do tbh, and they actually did it.