Why do weather forecasts get less certain farther into the future?

Forecast models start with observations from satellites, balloons, radar, and surface stations. Because the atmosphere is a chaotic system, even tiny uncertainties in temperature, humidity, or wind can grow and change the outcome. This is why forecasts are usually most accurate for the next few days and less precise a week or two ahead. Modern supercomputers and better data have improved forecasts a lot, but they cannot remove the inherent unpredictability completely.