There were plenty of collisions in the early solar system. There’s reason to believe that comet collisions were probably what caused elements other than hydrogen and helium to get to Jupiter. But sizeable impacts inside the solar system are now a pretty rare event, with one occurring only every few centuries.
In fact, the first observed collision of two solar system bodies took place only in 1994. The collision took place between comet Shoemaker-Levy 9 and the giant planet Jupiter between July 16, 1994 and July 22, 1994.
