Why Atheism instead of agnosticism of some sort

[Welsh cake]

My question to the atheists is why atheism instead of agnosticism of some sort?

Because agnosticism addresses the separate issue of what is ultimately knowable not what one's position is regarding belief or non-belief in deities, both atheists and theists alike can be agnostic, there are many Venn diagrams that illustrate this well.

[Lethe]

I'm an atheist because I lack a belief in the existence of deities, and for no other reason. I could call myself an agnostic atheist, but I'm technically agnostic about everything... perhaps.

The problem that arises with the "first cause" argument, other than the fact that it's highly unlikely there was one, is that it's even more improbable this said first cause would be sentient, as self-awareness is developed, not innate. A deity like Chaos, a non-anthropomorphic non-sentient deity would seem more reasonable in comparison to others, such as Yahweh, Ganesh, Venus, Loki, et cetera. An even more ridiculous claim is that there is a single anthropomorphic, sentient deity with a biological sex that lit the proverbial powder keg fifteen billion years ago, set his sights on an insignificant speck of rock around a single star in a single galaxy, twiddled his thumbs for another 4.5 billion years and then went "Lol, dem hyoominz gun beh trolled nao."

... I have to stop looking at lolcats.