Rav Hirsch and many other meforshim point out that in order for Adam to learn to appreciate Chave he had to first understand that he was unique and different from all the animals. Then when Hashem created Chava by splitting him apart. Other reasons also apply.
And God created man in His image; in the image of God He created him;
male and female He created them.
AGADAH: THE REASON WHY MAN WAS CREATED ALONE
QUESTION: The Mishnah states that Hash-m created man (Adam) alone in order to teach that one who destroys a single Jewish soul is considered to have destroyed the entire world. Likewise, one who saves a Jewish soul is considered to have saved the entire world. The Mishnah says that another reason for why man was created alone is so that a person will not say to his fellow man, "My ancestor was greater than yours." The Mishnah gives a number of other reasons.
The Gemara (38a) cites a Beraisa which adds more reasons for why man was created alone. One reason is so that evildoers will not say that they are evil because they are descended from an evil person (and use this as an excuse not to repent), and so that Tzadikim will not say that they are the sons of a Tzadik (and use this as an excuse not to avoid temptation to sin).
The YEFEI MAR'EH has difficulty with the entire discussion about why man was created alone. All of the animals also were created alone, just like man. Why, then, should there be any uniqueness in the fact that man was created alone?
(a) The YEFEI MAR'EH answers that at the time of Creation
there was a need for more men to be created in order to settle the
world. Had more men been created, the world would have been
established without Kayin and Hevel having to marry their sisters.
However, because of the reasons given by the Mishnah and Beraisa, it
was deemed more appropriate to create only a single man.
(b)The MAHARSHA argues that this is not the point of the Gemara. The
Gemara is not explaining why other men were not created together with
Adam ha'Rishon. Rather, it is explaining why Adam ha'Rishon was
created alone and only afterwards was Chavah created from him, unlike
the creation of all of the animals, where the male and female were
created as separate entities at one time.
According to the Maharsha, what does the Mishnah mean when it says
that man was created alone (i.e. without Chavah, who was created from
him only afterwards) so that a person will not say to his fellow man,
"My ancestor was greater than yours"? Even if Adam and Chavah had been
created as separate beings at the same time, everyone would still have
the same ancestors!
The Maharsha explains that the Mishnah means that had Adam and Chavah
been created at the same time, a person would have been able to say
that his traits come from Adam and not from Chavah, while his fellow
man's traits come from Chavah, or vice versa, as a person's traits may
emulate those of one parent more than the other. A person would say
that one of the two -- Adam or Chavah -- was more important, or better
than the other, and that he resembles one of them. Since Adam and
Chavah were created together as a single entity, no one can make such
claims, since all human traits ultimately come from the same source.
(The TIFERES YISRAEL seems to include both explanations of the
question in his understanding of the Mishnah.) (Y. Montrose)