The reason why you think I am still alive is that you are not told (that) I am dead.(时态必须一致,另：活于死,这里都是状态,而不是动作,因此都因该用系表结构）
or
That you think I am still alive is because you are not told (that) I am dead.
(that 作现行主语,谓语is.）
没用no one told you的原因是,主句从句的主语一致,更符合英文习惯,读起来更顺.
不用consider是因为,这个事情不用有理智地去思考,think含有主观的认为含义,think更恰当.