There are plenty of good reasons why Gods would be impossible. There is a natural order to existence that has to always be from simple to complex over time. Nothing begins as a complex or finished product without going through a series of simple steps over time to get there. Energy is the basic building block found in all of existence, no matter how simple. Energy also cannot be created, thus just exists as an uncaused existence.
Then there are the natural limitations that all existence must have, thus a God with limitations contradicts what our imaginations want a God to be, unlimited. There is no way around the limitations of existence and like the natural order another contradiction to what humans would like to imagine a magical super Being as being.
Finally there are the contradictions and paradoxes even trying to define a God because nothing can be everything without these contradictions and paradoxes occurring. When you add to this the apparent human ignorance found in all religions, the very poor reasoning of theists attempting to fabricate such a contrary, magical super Being, the whole concept becomes like that of a square circle, where we certainly have no evidence of their nonexistence, yet we can know why they are impossible through "good" reasoning. If you are wondering about the no evidence, that is because nonexistence cannot leave evidence to share. Which is why the burden is supposed to be on those claiming existence since actual existence can leave evidence to share.