Weather forecasters are sometimes wrong because you can’t predict weather perfectly. Weather is unpredictable.
We can predict weather up to 14 days in advance accurately. In 1980, it was 1 day, so we’re lucky. (I know AccuWeather had/has a feature where you can see the weather up to 3 months in advance, I wouldn’t recommend it if you’re gonna use it for weather-related planning or something. It’s pretty cool though if you want to see how accurate it is 3 months from now though)