Because MC is everywhere and MC is life! I'm from Europe and people there getting more fat too. My girlfriends sister moved to USA 10 years ago and she said that people are mostly fat because you have in USA tons and tons of fast foods. The meal there is very cheap in opposite against restaurant even against food you make at home.
Americans are also very lazy and they just pay a few bucks to get finished meal, than cook meal by yourself..

No, I dont think this is the main reason
You have to work out and keep your muscles busy in order to consume your energy
If you are taking a lot of energy (proteins & carbohydrates) you will start to grow more powerful but if you stop consuming this extra energy these joules will get deposited in the fatty layer