Why don't we say a pair of shirts?

Okay, you guys listen. We call jeans a pair, we call glasses a pair, we call earrings a pair, we call socks a pair but why don't we call t shirts a pair. Has anyone ever thought about this? And shirts have 2 sleeves (just like jeans and leggings have 2 parts for your legs) so why don't we say a pair of shirt.
Why don't we say a pair of shirts?
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