I am the owner of a small game studio, and when I read stuff related to the business side of the game, I often encounter sentences like: "with offices in Singapore, London, bla bla bla" or "they have just opened a new office in UK"...

Do big studios have separate teams working on different games in different cities/countries, or they just open an office in a specific area to benefit of some infrastructure or maybe of some investment opportunities? Why wouldn't they have a single big office in one city only? Or maybe it's because it's hard to drive talent/hr in one city/country?

I've seen QA-dedicated offices opened in less game-development centric areas, where unemployement was fairly high. According to local policies, and because of the local needs (and sometime local government aid) it was fairly easy to open up in these areas with a very small cost of operation (including cheap salaries).

What we could view as cheap labour, by our standard, was actually landing a job or not for them, so its always better than nothing, and even cheap pay, by our standards, was good pay for them.

Certain legal partnership make it also easier. Say you want to publish said game in a specific market because its legal there, and not anywhere else (for example: real money gambling), you're likely to draft the paperwork to get in there. While you're at it, mobilizing a few people to ease relationships with these will cut on actual travel expenses over the year. In the end, you might end up hiring some additionnal production staff and increase the roster of this office.

While not initially part of the plan, its a logical evolution.

Also, in areas such as where I work, there's simply no more workforce available for our needs. Currently, HRs from our top 4 studios are dealing with overseas pros which they have to pay relocation fees for, etc. It has reached a point where, having offices overseas would almost pay for itself...

I can see many other reasons to have offices overseas, but none that would happen until you've reached at LEAST 200-300 employees locally (unless your distribution method or legal dept. demands it)