People can say what they want, having lived all over the United States and also in Morocco for 4 years I can tell you that the bystander effect is much stronger in the United States than in Morocco, although there is some variance from place to place within both countries. I will refer to my experience in Morocco to provide some contrast.

Overall in Morocco people are in your business, whether it is to your benefit or to your detriment and this goes for everyone, not just foreigners like myself. America has a more individualist culture and it has always been this way although it has trended further in this direction in the past 100 years. I think there are likely a few factors that influence this.

I would imagine this comes from 1.) the culture brought here from Western Europe which also tends more towards individualism. 2.) A lack of homogeneity that would facilitate a sense of shared values as a collective 3.) The sheer size of the country which allows people to live in more isolated communities and housing that is spaced far apart 4.) Wealth and development that facilitates this separation e.g. you can live 20 minutes from the local market because many people can afford a car and there is a highway to drive it on.

Contrast that with Morocco where 1.) The population is largely homogeneous i.e. Arab and Amazighy Muslim 2.) The country is less developed economically so homes are built close together and literally on top of one another out of necessity which of course creates a physical closeness within communities 3.) The values of Islam guide day to day practice and while there are drawbacks as with any religion, in general they strongly promote acts of kindness and charity

In my experience, small town folk in the US tend to be more willing to help and more likely to intervene. As others have said big city types are numbed all forms of spectacle including emergency situations although I would still be shocked if I were to say crash my car on a busy street in LA and not have people come to my aid. But these are the big emergencies. Day to day smaller things do get largely ignored. There is a reason for this.

Americans value privacy and self determination. People might be reluctant to make the connection between the individualist values and a bystander effect in a real life emergency situation, but my opinion is that if you are less likely to insert yourself into other peoples' lives on a daily basis, to talk to them, to physically touch them, you will also find that social/physical barrier difficult to break in an emergency. It is taboo to assume. People hesitate to involve themselves out of respect for privacy and a plain lack of practice in it.

When you walk down the street in Morocco you more or less are socially obliged to say hello and how are you and how is your sister and how is your health and your child and so on... When you go through a crowd in Morocco you might put your hand against somebody's back or chest to gently guide them, an old woman might just grab onto your arm to help steady herself on the bus, these things are not unheard of in the US but breaking the touch barrier can be a big deal and in Morocco these things are done without a thought. The difference between you and me is just thinner, and that has its benefits and its drawbacks as anyone would tell you.