Because when a group of people is never represented they stop being in the public consciousness, and they stop being significant for many people. If the only time you see Africans is in ads for charities or news reports about the starving, you see all of Africa as a problem, and ignore the fact that it is a continent with many different countries, economies and lifestyles.
Similarly if old people are always portrayed as lonely or needy, or in care homes or hospitals instead of workplaces or at the heart of families, that is how they will be perceived.
Better to show a range of different ‘types’ of oldies, as is happening with other groups such as gay people or black people.
White thirty/forty somethings are portrayed as good, bad, indifferent, rich, poor, criminals, police, etc etc. As long as they dominate the media it will be their (albeit diverse) views, needs and concerns that will dominate society.