一个关于表语从句的问题

Question

The reason she has been successful is that she never gives up.
请问为什么不用because，如，
The reason she has been successful is because she never gives up.
我记得表语从句有用because的，如，
I think it is because you are doing too much.
请教各位了！

Answer 1

这句话完整的写法应该是The reason why she has been successful is that she never gives up.
the reason why ... is that 是定语从句中的固定用法,记住就行了。

Answer 2

The reason she has been successful is that she never gives up.
因为：the reason ... is that 是固定句式，that 引导表语从句
我记得表语从句有用because的，如，
I think it is because you are doing too much.
对此句正确。

Answer 3

the reason 已经表示原因了，再用because不是重复了吗？
句子的意思是“理由是...”，而不是“理由是因为...”，不通了。