The level of Tuxedo?

[artificer]

I have always seen this, my question is why? if you wear a 3 piece suit (with vest, Waist coat) you still wear a belt...

so why not wear a belt with a kilt and waist coat?

I think the major difference is that, with a 3 pc suit, your belt is roughly 1" wide, with a non-descript buckle.

With a kilt, your belt is+ 2" wide and tends to be paired with the waist plate for evening. The buckle overlaps the waistcoat.

Also, a kilt belt shouldn't (by design) 'keep up the kilt' it's more for a visual break between the halves of the body. A trouser belt DOES, by design, keep your p@nts up.