I do not claim to know everyone’s reasons for rejecting the existence of thousands of gods. let’s say a believer rejects other gods not because they’re absurd or due to lack of evidence, but because they believe the existence of their God rules out the existence of other gods. An example would be If the God of classical Christianity exists, then by definition Allah can’t exist. There can’t be two all-powerful gods in the same universe, as they would limit each other’s power.
So if that is why the believer rejects other gods, then this doesn’t help explain why he should reject his own god. If the believer rejects other gods because they are creatures of ridiculous ancient mythologies and because there is no good evidence for their existence, then this should help them see why they should reject their own god, who is also a product of ridiculous mythology for whom there is no good evidence.
Let’s say someone argued that because science can’t explain the undersea bloop detected in 1997, it was probably caused by the supernatural action of Rongomai, an ancient polynesian whale god. Would someone of a different faith take this seriously? Of course not. Someone of a different faith will quite happily argue that because science can’t totally explain consciousness, it was probably caused by the supernatural creator god.
The double standards of the believer can be even more striking in reality than in the above hypothetical example. If someone claims they don't believe in magic, and yet they believe in the supernatural creation by a divine being, an event which is literally magic, even according to 99% of theologians!