Can someone tell me why Vietnam has so many bikes?

You know, it's kind of interesting. In South Korea, they largely use cars, but many of the roads they have aren't too well suited for lots of traffic.

Koreans are very, very aggressive drivers, and if you leave enough space between your car and the sidewalk, the ones on a motorbike will drive right between it (I once saw a motorbike carrying 4 people, when it looked like it could only hold 1 person comfortably). The traffic conditions, especially in the cities, are comparable to places like Los Angeles and New York, since they have so many people living in such a small area of land.

Heck, I remember one time my buddies and I were taking a cab back to out post. We were driving down the road, and in front of the cab driver we saw an "open" slot, which was quickly being closed between a bus in front, and a truck in the other lane. The cab driver quickly sped up, and then drove through the spot between the bus and truck. And as he did that, I swear we all almost collectively felt our lives passing before us, thinking we'd get crushed between the two vehicles, and bracing for impact. Luckily he was a skilled driver, so we managed to bypass the truck and bus (no, we didn't tell him to go through, he did it on his own).

As for Vietnam, yeah, I'm going to also say that it's because bikes are much cheaper than cars, and that they're probably more well suited for that area than a car is (Vietnam probably has narrower roads than in the US I bet).

Jam it back in, in the dark.