Children are also wealth power and influence especially if they are very numerous, in fact polygamy was abolished in the US because Mormans were using this and many children to take over whole states like Utah and thus major political powerYou answered it. People lack the knowledge and do questionable things, especially when they are poor and have too much time on their hands. It's easy to blame religion, but there are comprehensive explanations for this.
Throughout history, the reason people had many kids was to ensure that few of them pass a certain age to grow up because the child mortality rate was high. The same countries today that have high birth rates have a high mortality rate.
The second thing can be that people have lived in that condition for so long that it has taken deep root in the culture. This is not a Muslims conspiracy.