it must have been raining when he got back home ,for he was wet all over

问题描述:

it must have been raining when he got back home ,for he was wet all over
这里为什么不能用must be raining 要+been 是被动的意思?

这里为了强调下雨的时间在他到家之前,也就是在他回家的路上
get back home这里作为一个时间点,完成时是对于这个时间点之前的推测那用having raining 可以否?