The answer is, it all depends - on some of my kilts the belt loops are at different heights - on the kilt I'm wearing at the moment the back loops extend above the waistline and the front ones are 'as normal' - I need to shape the waistline of kilts by an inch and this was my first kilt.

It had a belt but no belt loops at first, however, when I'd had to go back and pull it off a bush twice, it got the loops. Safety pins were just not 'man' enough out in the wild back garden here.

If the kilt extends above your narrowest level all way round and is self supporting then a belt should stay put, and so should the kilt underneath it with no need of loops.

I susupect that most garments distort to some extent at the waist, it is just not obvious, though belts often take on a distinct curve or twist after a time. The wide belts worn with kilts would tend to resist the deformation more than an ordinary narrow belt - putting strain on the loops or even on the body. If I wear a belt 'low slung' I get a back ache which is instantly eased by lifting the belt half an inch.