Why is 'shirt' singular but 'pants' are plural?

One could argue that they have two separate areas for one's legs to go. Or because legs are themselves two independent objects that the pants fit the plural. ... A shirt is just pants for your top half, and that's a singular object, even though there are holes for each of your independent arms in which to go.

I don’t know, why does a lamp have a shade, if you need light why would you need shade?

If you look at adverts for expensive clothing, they like to call a pair of trousers a "pant". It's like when they call a shirt a "shirting". Idiotic attempt to make customers feel better about paying too much.

Because a garment like a shirt is a single garment , whereas pants and leg covering were originally two separate garments. Thats why  you wear 'pair' of knickers but its now only a single item. 

Some things in the English language can't be explained!

because pants have two separate legs which in the past were different (one for the left and one for the right) but some one joined them together into one unit. 

Because a pant only covers one leg.  Don't argue it with me, I didn't write these rules.