Why isn't the fit perfect if all the continents used to be Pangea?

The fit is not perfect because of several natural factors. The main factor responsible is erosion. Pangea is considered as a super continent that existed long back. After that the continents got separated. Now they do fit because the continents are surrounded by water mostly on three sides at least. the erosion due to water has destroyed the initial shape or formation that was there when it separated from Pangea. This is the reason that now the shapes do not fit together to form the same Pangea that existed long time back.