Generally, saying "that is why" in response to a "why" question is already understood, so it might sound rude. However, if you are explaining a bigger subject and it needs to be clarified in multiple steps, saying "that is why" is usually said in the middle of a paragraph and it might start the sentence. Examples: "Why are you going to the concert with Susan?" "Because I like Susan, that's why!" <--- Understandable, but "that's why" is unnecessary. "Why do you always make me eat my vegetables, mom?" "When you went to the doctor last week she said you needed more healthy food in your diet. That's why I've been giving you more steamed broccoli at dinner this week." <---This adds context to the conversation and explains more than what was initially asked.

"its because" é como "...é porque..." "thats why" é como "...eso é por que..."