You are right to an extent that some general governing laws must be made for businesses of different countries to trade, however, when it concerns Tax laws, or Excise or Customs Laws, it has to be country specific. This is because different countries has different taxing systems. Also Countries have different priorities of exporting and importing goods and services.
For example, for one country it may be beneficial to import an X product so it may make rules to make such goods import tax free which may not be beneficial for another country so it may make rules to levy taxes on such products. Same applies for exporting.
So, it is difficult and also not advisable for Governments of different countries to come up on an agreement on this.