Because you can run but you can't hide. If you institute protectionist policies, you will in turn become less competitive. Hence, you will require even more protectionist policies, that in turn will make you even more uncompetitive. In the end, you will have higher prices for lower quality domestic products.
I don't like gravity, but it doesn't mean I can ignore it and jump from tall buildings. While economics is not an exact science (probably not even a science), you cannot ignore its principles.
If protectionist policies worked, we would have figured out by now. The truth is that limited protectionist policies might work to give a push to a nascent industry in the short to medium term. But it does hurt the country as a whole in the long term.