It's not the case with 〜ではあるまいし
Grammar: Reason and Cause » 〜ではあるまいし
〜ではあるまいし expresses a cause or reason. It follows a noun or phrase, and is used to say “Because it is not ~” or “Because you are not ~”.
In the case of expressing a reason, a criticism, warning, advice, or an order may follow after し. However, it’s often the case that the portion after し is omitted.
When following a verb, ん or わけ may appear before で.
The colloquial counterpart is 〜じゃあるまいし.
Seeing a frozen puddle on the ground, Nishikata is excited at the thought of picking it up and holding it. Takagi asks if this is what he’s thinking, and he quickly denies it. Evidence piles up for Ran that Conan, the smart young boy staying with her family, may actually be Shin’ichi, her missing childhood friend. She’s convinced that he’s Shin’ichi on the inside, but the sticking point is how he went from being a teenager to a child. Shin’ichi’s teenage friend Ran enters Professor Agasa’s house looking for him, unaware that he was forced to take a poison that inadvertently turned the teenager into a young child. When Ran sees the boy and asks his name, Shin’ichi recalls Agasa’s suggestion to not reveal who he is. Panicking, he gives the name “Conan”, explaining the oddness of it by saying his father is a fan of author Arthur Conan Doyle.Further Reading
Examples
It's not like I'm a grade-schooler with 〜じゃあるまいし
It's not like this is science fiction with 〜じゃあるまいし
It's not like you're a foreigner with 〜じゃあるまいし
