I can understand " a pair of socks", " a pairs of shoes" and "a pair of gloves" because there are 2 but why do we say "a pair of trousers" "a pair of pants". I know there are 2 legs in them but there are 2 sleeves on a tee-shirt but we dont say "a pair of tee-shirts"
We say 'a pair of scissors' too. The reason for 'a pair' of pants or trousers is that we thought of them as two parts.That's not as silly as it sounds. A long time ago, the garments were in two separate parts, tied together at the top. We don't , or didn't, say ' a pair of doublets' because the doublet was made as one.