B737 First Officer reported a critical ground conflict while on short final approach with an aircraft that was taking off in front of them but had not yet left the ground. Crew elected to go around and advised ATC of same.

Narrative

These altitudes are estimated based on the approach and my memory of the events. We were on final approach when we checked in with tower (est 2000 AGL). We were initially given clearance to continue; but not to land on XXR. At 1000 AGL; we were still not cleared to land (we have a callout at this altitude; so I am confident of this). Somewhere inside of 1000 AGL we were given landing clearance (est 800 AGL); but we could see that there was a commercial aircraft on the runway in the hold position on the runway. The commercial aircraft began their takeoff roll after taking the runway; but it became apparent that they would not be airborne in sufficient time for us to land safely. I do not recall the exact time that they took the runway or began their takeoff roll. We performed a go-around at approximately 100 AGL because they were still on the runway. We did not believe that they would be airborne in sufficient time for us to land. After informing tower of the go-around; we were given a 30 degree offset heading; and I was able to maintain visual on the departing aircraft.