Because they are either fallacious or incoherent in one way or another: AB has provided no end of examples in this very thread.
Unfortunately no counter arguments have been forwarded by your side which do not end with "we don't know" or "science will one day show".
Antitheists seem to be given to inventing fallacies and straw men vis Courtiers reply, Going nuclear etc. Which just cover ignorance and show special pleading.