Because most fish cannot fly or hop, skip and jump continents due to natural barriers that restrict distribution. Things like oceans, mountain ranges and suitable habitat availability tend to restrict natural Brackish / freshwater fish movement... That is why. Fish can not easily move across oceans and continents without humans being the vector of introduction.
Are you implying that it is just fine for Invasive and ANS species to continue to be introduced Just because people have become the most invasive species on the planet? Good for us so good for everything else....
Sorry... it is not OK for these animals to persist where they do not belong. The Great lakes as well as the Mississippi drainage etc. (Pick a North American watershed) have enough problems already due to Anthropogenic effects.(damming, channelization, pollution, siltation etc.) we do not need to compound it with continued destabilization by invasive and exotic aquatic life.
The above Michigan law placing big fines on the movement or possession of Invasive species is one of the best in the country IMO..Unfortunately it does not go far enough to jail those transporting these fish it just fines them.