They have no land to farm and they can't buy what they need to eat. Right now, almost half of the people in South Asia and Sub-Saharan Africa live in such absolute poverty. Poverty makes people malnourished. In its turn, their malnutrition impairs their performance in school and work, and makes them more vulnerable to disease. This makes it harder for them to get good jobs which could free them from poverty.
War breaks up the farming cycle and the marketing and distribution of the farmers' harvest.
Worse, in far too many situations, food supplies are plundered or stolen, and never reach the people who need them the most. In some countries, even if fighting never occurs, heavy military spending drains money away from food production, education, and health care. Global military spending is more than the total income of the poorest 45 percent of the world's people. Many governments actually hurt their country's own food production by favoring the interests of the cities or industries or large landowners over the interests of the rural areas and the small farmers. People born in 1900 were born into a world with fewer than 2 billion people.