Why is climate so much harder to predict than weather?

The reason it's so hard to predict the weather very far in advance is because weather is incredibly complex and dynamic. Factors like today's temperature, humidity, prevailing winds, and local geography all have an influence on tomorrow's weather. What happens tomorrow determines what will happen the next day, and so on. Even the tiniest unknown factor in today's weather, say the humidity over a patch of forest, increases the uncertainty of making tomorrow's forecast. The situation makes the forecasts for next week even less certain, and forecasting even further into the future becomes increasingly more challenging.

Climate is very different. While climate is also an exceedingly complex system, we aren't looking at local, day-to-day details, but rather focusing on the average conditions for a region over time. And those change much more slowly. Nobody can tell you what the temperature will be on August 10, 2020, in New York City, but it's very likely to be a lot warmer than on February 10, 2020, because New York City's climate is hot in summer and cold in winter. The city also happens to have a fair amount of both rain and snow. The climate in San Diego, California, is warm, but not hot most of the year, with relatively little rain and essentially no snow. Buffalo, New York, is very cold in winter and gets a huge amount of snow. And so on. You instinctively know the climate where you live because it doesn't change a lot from year to year.