You are probably right, but I suspect there's more to it. Having made a knife from scratch I can tell you that a good guard is not that easy to either design or affix securely using traditional (primitive) methods. It can be done, obviously, but it adds time and trouble. Older (before 1800 or so) knives, as opposed to swords and daggers intended for use in combat, didn't have much in the way of guards. I suspect this was because it was easier and quicker to make them without guards.

And don't forget the ease of carry issue - it is much easier to tuck a hiltless knife deep into a soft leather sheath and have it stay there than it is to secure it in a sheath that holds it by the blade only because the guard needs to be accommodated.

True, these knives weren't typically being used for heavy stabbing or chopping - there were other implements for that. It is also true the steels and thicknesses of these older blades were such that putting too much strain on a blade could bend or break it, so they weren't used the way some people use knives today.