Why are men allowed to marry multiple wives in Islam?

This is not a thing particular to Islam, but in Arabia and many parts of the world polygamy was and is normal. In Arabia there was not set limit to the number of wives one may have. Islam set a limit to four if they are treated justly.

A question was put forward to Ayatollah Sayyid Muhammad Taqi al-Modarresi about marrying multiple wives:

Question: I am interested in polygamy as I intend to marry a second time. It has to be noted that I am fair and just with my wife, and she does not fall short in fulfilling her duties towards me. We are remarkably happy with each other according all our relatives. Is it permissible for me to marry another woman if I fulfill all the conditions of justice and adherence to the divine limits?

Answer: It is permissible, but we advise you to think carefully before taking action to marry another woman because your current married life will be affected, and you are likely to lose the happiness that you enjoy at present. If you are living a happy life as you have said, then why do you want to marry another woman when you do not need to?