Why did weather replace it with that?

In English, "weather" is an uncountable noun, so it can't be used to refer to it. On the contrary, we can use "that" to refer to the weather, because "that" can be used to refer to the things or concepts mentioned above. For example, we can say, "It's a beautiful day today. This makes me want to go for a walk. " The weather today is really good, which makes me want to go for a walk. In this example, "that" refers to the aforementioned "weather".