God could have created through evolution is he had chosen to.
But he chose to do it by creating in six days as described in Genesis. It couldn't be any more clear in the Bible that the intention of the Bible authors was to convey the meaning of 6 days.
There are some serious theological problems with evoluton. For example it puts death before the Fall; and many passages tell us that death is the result of sin. If death predates sin then why did Jesus need to die? It makes a nonsense of the Gospel.
Evolution also implies that God's 'very good' creation was full of death and disease - for that's what we see in the fossil record, which Christian evolutionists put before Adam and Eve.
You will find many good articles here which explain in much more detail why many Christians reject evolution.
http://creation.com/creation-why-it-matters
Incidentally, people who say that 'evolution is proved', generally mean that they have evidence of natural selection and variation. Well creationists believe in that too!
Peppered moths, finches beaks, fruit flies, different breeds of dogs, and all the other textbook examples, are natural selection. They all have in common that they are downward, information losing changes. (Or information netural at best.)
The evolution of man from amoeba requires a huge net increase in genetic information. No mechanism for this has been observed. Evolutionists propose that mutations are the mechanism. However all observed mutations are information neutral or lossy. Yet evolutionists have the nerve to tell creationists that we are not scientific, when their own 'science' is contradicted by observation!
It is not quite as simple as speciation either. (Evolutionists often claim that speciation is proof of evolution.)
We label tigers and lions as different species. yet they can interbreed (producing ligers and tions). So clearly lions and tigers are descended from a common big cat ancestor. Possibly all cats alive today are members of the single big cat kind.
Remember that God created animals and plants 'after their kind'. This is emphasised repretedly in Genesis, where the expression 'after their kind' is used something like 10 times. Almost as if God knew that evolutionists would come along saying they know better than God how it all came to be :)
What we observe today is entirely consistent with plants and animals reproducing after their kind. God in his wisdom, created plants and animals with huge genetic diversity and the ability to adjust to different environments. The single big cat kind has given rise to lions that live in dry hot desert, to siberian tigers that live in very cold areas, to pumas that live in tropical rainforests. How cool is that. That's like writing a computer program that can reproduce itself and can adjust itself to unknown future conditions.
But note that this speciation is a downwards process. Lions and tigers have less genetic information than their ancestors. They are more specialised.