Why couldn't it be possible for God to create everything, and then allow things to run there own course after that?
That depends on what you mean by 'Creation'. A
literal creation cannot coexist with evolution because the mountains of hard evidence which back up evolution (transitional species, DNA, the fossil record, dating techniques, etc) don't come
close to agreeing with the idea of a 7-day creation and a 6000 year old earth.
As an example, I'll use my favorite creationist argument - one that attempts to explain why we never find fossils out of order:
During the Flood, animals that were 'less evolved' sank towards the bottom, and animals that were 'more evolved' were able to swim higher towards the surface of the water.
There are so many logical and factual errors in this statement that I could spend all afternoon just pointing them out. It is a rather common Creationist argument, but I'll give them a little credit: At least it isn't blaming fossils on Satan, or God playing tricks with us, or scientists making up every single transitional form in order to further their 'agenda'.
Now, Creation in the Deist sense, which is what you described, IS possible and entirely compatible with Evolution, Ambiogenesis, and the Big Bang... Though I feel Deism is a little contradictory, I see nothing wrong with filling in the blanks with God, so long as people are open-minded about the REAL evidence that surrounds us.