Why does the weather forecast say the highest temperature today and the lowest temperature tomorrow?

Because the time to report the weather forecast is usually in the morning, and the lowest temperature of a day usually appears in the early morning, when the weather forecast is reported, the time of the lowest temperature of that day has passed, so there is no "forecast". The highest temperature of a day appears at noon, which can be predicted, so when reporting the future weather, only the highest temperature of that day and the lowest temperature of the next day are reported.