"Why do you not believe in a God?"
This really depends what you mean by "God", but for purposes of answering I'll assume you mean a conscious entity - which is morally good - responsible for designing and creating the universe and capable of (at least) forming personal relationships with people and at the very least offering some kind of guidance. If these are not the things you mean, then my answer will change.
I do not believe in this because I think the idea is so incompatible with what we know about the world as to be literally impossible. It would have to hold that a morally good God quite happily designed the niagleria fowleri amoeba - which burrows into your brain and devours it slowly, and kills a few hundred children every year for no reason other than they were unlucky enough to go swimming at the wrong time.
It would have to hold that a morally good God separated the continents and thus knowngly prevented the indigenous peoples of the Americas from becoming resistant against diseases which were more widespread in Eurasia. This resulted in hundreds of millions of innocent people dying slowly and horribly when the Europeans eventually arrived, and is probably one of the most complete examples of the annihilation of an entire people recorded in human history.
That, for me, is the most persuasive argument against a morally good, personal God who is powerful enough to have any agency in the universe. It is fundamentally incompatible with even our incomplete knowledge of the world - and as we learn more, it has simply become even more so. Other arguments are more effective depending on what kind of god you are arguing about.