i think you are not very familiar with the concept and theories of Taxation/Revenue ...
To make you understand in short, the good things u r bringing in Pak would use Paki resources, like your flashy cars will use our roads which have to be maintained by the government for which it needs money and hence it gets that money at a fixed percentage from everyone who is using these luxuries on government provided resources. The money you pay at airport is supposed to go to the government treasury and if it doesnt then our implementation of the taxation system is at fault and not the theory or concept of taxation. Hope this will clarify some confusion in your mind.