Your thoughts on less people in Africa would be true for previous outbreaks (which is why they were easily contained) BUT this time it has gotten in to cities that are every bit as dense as Americans if not more (part of the reason it has gotten so bad). But it will not spread so quickly over here for several reasons:
1. General hygiene and sanitary conditions are so much better here.
2. Most Americans trust the healthcare system and will listen to their advice; also hospitals will save many more people here so everyone will realize that their best chance to live is to go to the healthcare system immediately after feeling symptoms which means they will be isolated very soon after becoming contagious. In Africa in many cases seeking the healthcare system is simply going away somewhere to die so even some educated and most uneducated people think whats the point Ill just stay home. This causes them to infect many members of their family and if they die at home their dead body is super dangerous.
3. I have followed the Ebola outbreak closely for several months to the point of reading African newspapers (oi vey), and there is something that I can't quite understand but is very real. There is a huge stigma on Ebola and many even educated people, who should know better, refuse to acknowledge that Ebola is real and around. One of the nurses who got it in Nigeria and eventually died gave it to her husband (possibly fiancé I dont remember) who got very sick, but survived. I saw and interview with this guy and he totally refuses to believe he had Ebola and you can tell he believes it to his core ... He even totally accepts that his wife had it and he got sick after tending to her but cannot even entertain the thought that he had Ebola ... he was a university graduate and worked in marketing for an oil company so we're not talking about some dude in the bush with a bone through his nose. Its almost like some of the people think if they just refuse to accept that they are getting sick they won't. Needless to say this causes them to go around and expose many more people.
4. In the US culturally we have little contact with dead bodies, often they will die in a hospital and then they are taken to a morgue. I don't think there is even such a thing as a morgue in Africa. People bury their own and tradition is that the family washes the body and people kiss the corpse at the funeral. The body of someone who just died of Ebola is massively filled with the virus and is the most contagious thing there is. But again there is the stigma and all kind of people try to hurry up and quickly bury their family members because they refuse to accept that they died of Ebola and its almost like if they can bury them before the crews come to test them they won't have to accept that they died from Ebola. And if they dont accept they had Ebola they wash the body and kiss the corpse.
The stigma is weird this one village refuse to acknowledge Ebola was there, but were super pissed off because they were certain someone had poisoned there well causing all these people to die. I dont doubt this guy from Liberia didn't think he had been exposed the family told him she didn't have it and he believed them because he didn't want to believe otherwise or just couldnt. Now he has spread it here, so you see what a problem this is.