Most of the countries making up the 3rd World, were colonies of some other power. During the years of that colonisation, society was set for a population. When the colonisers left or were driven out, the structures left behind were adequate for that population.
Most 3rd World countries do not use birth control, ban abortion, and babies are produced at a rapid rate. There is no shock that an uneducated unemployable will have five children before she is twenty.
As there is little investment in schools or hospitals what was left by the colonisers to manage 1.4 Million people remains to deal with 3 Million people. In most cases, it remains as it was left.
If the school was built in 1957 for 1000 students, the Coloniser left in 1964, and today, with little upgrading, if any, that old school is to operate with 2000 students or more.
If this hospital was built in 1912 unless the colonisers maintained it before they departed in 1962, that hospital is as is where is, but expected to manage five times the population it was built for.
Simply put, there are too many people for too few places.
Wise leaders would have recognised the situation early, implemented free abortions on demand, adult education, and focused on maintaining the infra structures left by the coloniser and creating new schools and hospitals. But there are few wise leaders in the Third World.
Many get into politics to fill their pockets. If they are honest, they often appoint dishonest people to various positions. Many of these people may be unqualified, disinterested, and focused on filling their pockets.
If the Chinese paradigm was implemented, (one child per family) than many of these nations would be able to manage and develop their economies.
Of course, this won't happen.