Why doesn't the weather forecast predict humidity?

Humidity changes very quickly. Once my thermometer showed that the humidity was 90% (it was summer), and it became 70% after a minute or two. It's not that my thermometer is bad, but that the moisture in the air changes too fast. Even if the average humidity is used, the data cannot be expressed intuitively and rigorously. Besides, it's a weather forecast, and it can't predict the future weather. The weather on the mobile phone shows the humidity, but most of them are inaccurate, and the correct rate (the difference between the actual humidity and the predicted humidity is 5%) is only about 40%.