The problem is that the pre-supposition that "Mankind is all there is." is made a priori to the conclusion of "There simply can't be a god because mankind is all there is."
I don't know who you're thinking is making this conclusion. I'm certainly not, and I highly doubt any competent scientist would. The idea that gods don't exist came about because of the lack of evidence for such gods, nothing more and nothing less.
There is no point to such a pre-supposition aside from scientists fearing the easy way out by saying "God just did it."
That isn't the "easy way out", that's the lazy
way out. Claiming that a god did something without investigating it is nothing but an excuse. It wasn't even particularly excusable in ancient times; it certainly is not excusable now, with our far superior level of knowledge about the way the universe works.
and if they are already secular enough to believe God doesn't exist at all I see no point in writing off a creator prior to observations in the first place. It only serves as a tool for scientists to write off such explanations as 'non-scientific'. It is pretty much just a means to an end for secular science to dominate the playing field while they can just write off non-secular science as merely 'not science'.
You really don't understand why scientists refuse to make the assumption that a god or gods were responsible for something, do you? The reason is because scientists want to figure out how something happened. Saying that a god did it, without evidence, doesn't tell you anything about that at all. It would be like if I put a computer together for someone, and someone else asked how the computer was put together. What good would it tell them that I had done it, especially if they had no way to get in touch with me to ask me how I had done it?
It's a hundred times worse than that when you start claiming a god was responsible for something, because that's an a priori
presupposition too, used to justify the existence of a god despite the lack of evidence pointing to one. So scientists don't make either - they don't assume either way. They just try to figure out how things happen, and use what they discover to draw conclusions about what might have caused it.