Why No Underpants with Kilt?

It is unknown exactly why the practice of wearing no undergarments under the kilt began, but many sources indicate it originated with the Scottish military uniform in the 18th century, leading to the invention of such expressions as "go regimental" or "military practice" for wearing no underwear.

Why don't you wear pants with a kilt?

Not wearing underwear under a kilt is a Scottish military tradition. ... "Generally, wearing a kilt is better for your health because you're not constricting that area and the air is able to flow around. "I think true Scots who like to wear kilts a lot will wear underwear, it's just more hygienic.

Is it offensive to wear a kilt if you're not Scottish?

In the true sense of the meaning yes, but as long as it isn't worn as a joke or to make fun of Scottish culture, it's more cultural appreciation than cultural appropriation. Anyone can wear a kilt if they choose to, there are no rules.