Why is the default set at birth? The default at birth is that people can't walk, we don't consider that people in wheelchairs have 'reverted to the default'. We consider the default is that people can walk.
We don't take 'at birth' as meaning 'natural' for most things, why then should it be for atheism?
There are many scientists who suggest that humans are predisposed towards believing in 'god(s)', and that if a child grew up on a desert island then they would most likely believe in god(s) (obviously not any specific Religion's god(s) though)
An example:
"These results suggest that the tendency to view nature as designed is rooted in evolved cognitive biases as well as cultural socialization... Results from Study 1 revealed that even though religious participants’ baseline tendency to endorse nature as purposefully created was higher than non-religious participants’ tendency to do so, non-religious participants also increasingly defaulted to understanding natural phenomena as purposefully made in the being-made group when they did not have time to censor their thinking."
The divided mind of a disbeliever: Intuitive beliefs about nature as purposefully created among different groups of non-religious adults, Cognition, Issue 140, 2015
http://www.bu.edu/cdl/files/2015/04/Creator-online-publication1.pdf
This suggests that it takes cognitive effort to view the world atheistically, rather than the other way round. Why
must people accept, with no evidence, that the default
must be atheism?
The scientific consensus on 'whether or not humans are predisposed to believe in god(s)' would be that it is an interesting area that warrants further research.
The fact that almost all societies in human history have developed gods/religious concept is also pretty decent evidence that religious beliefs may be the default.
So we get the arbitrary statement that the 'default' is set at birth, and that atheism therefore remains the default for the rest of someone's life, even though there is scientific debate as to whether or not religious beliefs are hardwired into us.
This view is presented by people who consider themselves 'rational', yet they consider it axiomatic that atheism must be the default because 'it just is, ok' and deny that they are making any value judgements amidst numerous unsupported assumptions.
Saying atheism is the default requires assumptions about babies minds, what constitutes a default and whether or not 'god' is hardwired into us (amongst others).
Whether or not atheism is the default is something I personally lack the evidence to support, so I'll not profess any judgement on it.