he could hardly believe what he saw 为什么是what that

what he saw（他所见到的事）是一个由疑问词what 引导的宾语从句，作believe 的宾语。不能用that, that 引导宾语从句时只能用于从句是肯定句的时候。
如：He could hardly believe that he had won 5 million dollars.他简直不敢相信他中了500万美元。
i don't know where he lives.(where 引导一个特殊疑问句）

what这里是专指说话内容
that后面应该跟一个从句··
he could hardly believe that he saw his own son is selling drug``之类的

这句其实是这样的 he主语 could hardly believe 谓语 what he saW 宾语,这个宾语从句由HE为主语SAW为谓语WHAT为宾语所组成.这话本身就是他看见了什么what did he see OR he saw what,你用THAT不能表示他看见了“什么”的意思,
还有就是
WHAT THAT 同是定语从句时有几大情况是不可互换的,可以问老师,我不记得了==|||

what he saw 是个短语，意思是 他所看到的东西。
而用 that 时，后面需要跟一个完整的句子才行，所以 he saw 不能用在这里。 要用的话，可以写成例如下面的语句：
He could hardly believe that he saw so big a god .
他几乎不能相信他看见了那么大的一只狗。