why is my Passenger seat belt getting wet when it rains
Because you have a leak somewhere.
If the vehicle has a sliding roof, it may be that the water is not going down the drain tube.
If you have installed an after market roof rack that requires drilling holes in the roof, that may be the problem.
Also, the trim and seal around the passenger door may be worn or damaged.
A good mechanic will find this leak quickly.
We cannot from here tell you what the ause of you leak is, but oinlky point you in the right direction.