These people are destined to live in a vicious cycle of extreme poverty. Illiteracy has many causes. In developing countries, some of these causes recur over and over.
Some are preoccupied obtaining their next meal e.g. begging, stealing, picking through garbage, child labor, prostitution—even slavery. Some live in remote areas beyond the reach of school systems. Some speak indigenous languages with few bilingual teachers and even fewer textbooks, never learning an internationally recognized language and leaving them without access to the world economy. Most live in households where parents are illiterate and thus unable to offer any assistance with homework.
Education is crucial but of little value without a job. When unemployment is so high and so few jobs are available, it's no wonder parents discourage children from completing secondary school—they can be more productive farming and helping out their families.
Teachers are often poorly trained. Most have never touched a computer.
Some experts believe that malnutrition can cost upwards of 15 IQ points, creating communities of developmentally disabled students. Attendance is spotty for both teachers and students.
Decrepit buildings, furniture, books and supplies continue to deteriorate due to lack of funds. Unlike other development efforts, providing free education to the extremely impoverished is not an easily sustainable enterprise. Jobs don't benefit schools except indirectly as bragging rights.
In many developing countries teaching pays too little. In some, nations cannot afford to provide free compulsory education. Parents have to pay tuition.
When children have to beg to eat how can we expect them to afford tuition, uniforms, books, supplies, and transportation?