Because it destroys the local industry which you will need at some point.

As simple example: the British supported the German chemicals industry prior to WWI.

At the start of WWI, the money the British spent had been turned into German chemical expertise. This lead to the development of the Haber process, poison gas, and generally making Germany a powerhouse in chemicals in the same way Britain was in mechanical manufacturing.

If a tariff had been put in place on German imports to Britain the German chemical industry would have been much smaller, with much less capability and WWI would have finished in 6 months because Germany would have run out of nitric acid for their explosives.