First I have to say that I am not a muslim and the muslim view seems strange to me. I agree with Gypsy. There are thousands, probably millions of unwanted and unloved children in the world. Adoption means that these children are given the chance to enjoy the love and support of a real family including all the priveleges that brings, including inheritance.

As far as my understanding of Islam is concerned, it goes something like this:

Before the time of the prophet Muhammad (PBUH) adoption was normal in Arab society. Muhammad (PBUH)himself adopted his former slave Zaid. Zaid then married the prophet's cousin Zainab bint Jahsh. The marriage soon experienced difficulty as Zainab was highborn and her husband was a former slave and she felt superior to him.

They divorced and Allah commanded Muhammed (PBUH) to marry Zainab and take her as his fifth wife. This presented a problem though as marrying your son's wife is haram. However, if it is decreed that the adopted son is not actually a son (in the true familial sense) then it is not haram.

Therefore, in marrying Zainab, Prophet Muhammad (PBUH) demonstrated that Zaid was not his son and that therefore the wife that Zaid divorced was legal for Muhammad (PBUH) to marry.

This is my understanding as to why adoption is not allowed in Islam.