Why build cars with 600-1200 horsepower? Because people buy them and you can make money selling them. Its business, and maybe, just maybe some game developers really love what they do for a living.

just because only 20 out of 100 people want a "core" game doesn't mean the other 80 people all want the same thing.  also, don't think of people as people, think of them as dollars.  I buy about 20 games a year while my mom (a decidely not core gamer) has bought a total of 4 games for her wii over 2 years: wii fit, wii play, endless ocean, and deca sports (and wii sports of course).

casual is maintream or just another market ?

Going by O-D-C's long, Mainstream = 80% of customers and Casual populace > Hardcore populace so yes.

Mainstream = Casual.

a core game is a game built exclusively for a certain group of people, look at a very traditional JRPG, (not Final Fantasy but something like... Ill say Star Ocean IV, but Im not a big JRPG player so correct me if Im wrong) the game is built for that market, it isint built for the mainstream, sure the mainstream can catch on, and in the begining years of many of these genres thats what happened, the mainstream caught on to JRPG's, FPS's etc, and they became big. And now maisntream games in those genres like Final Fantasy XIII and Call Of Duty: MW2 exist.

1. What are 'core' or 'hardcore' games? I would argue that for example MW2 and ACII are not casual titles, they have been selling well enough no?

2. As others have said, enthusiast/core/harcore gamers spend a lot more on their hobby.

3. A lot of people working in the games industry are huge game fans ('core'/'hardcore') themselves. To keep them at your company, you have to give them tasks that they like, and they want to work on these 'core' or 'hardcore' titles. I know I wouldn't like to work on Imagine: Babiez...

Mainstream is the most advertised games with tons of budget, right?

overall, they make those games because a market exists. If either 360 or ps3 were not in this generation, you could, imo, effectively add 75% or so to their console base and game sales.

I would say a mainstream game is one that attracts the masses, be it Wii Sports or Call Of Duty: MW 2, advertising helps reach those people, but in the end a mainstream game is one built to be enjoyed by all, not only fans of that particular genre.

games are made to be enjoyed by all, while there might be some niche games, rarely advertised, popularity has nothing to do with enjoyment. The best games or the most enjoyable are never mainstream. That goes with everything.

The word for mainstream = Marketable.