Taking a look in internet you can find photos of mutated mammals with 6 legs. Some cows with legs in unnatural positions were shown in website versions of widely known newspapers (you can easily guess here that if the animal gets his additional legs in the back or another unuseful position why a mutation like that wont spread, but the point here it's 2 additional legs in mammals do appear occasionally) . Also there are some dubious photos of sheeps with appparently functional 6 legs, these ones are dubious and hard to confirm if real, some clearly fake but the dubious ones are there and make you wonder. Insects have 6 legs, there are confirmed mutations of mammals with 6 legs, then why did it never appear a mammal species with 6 legs. Is there any known reason why 6 leg mammals are only found in mutations? (how about size and enviroment, is there any reason in those for them not to get 6 legs?)
If stability is the goal, more legs tend to be better. However, having lots of legs also comes with a cost: they take energy to build and use, have extra weight, etc, and they don't really add any speed.
Insects are incredibly successful and likely evolved from many-legged ancestors, losing legs for efficiency but stopping at six. For small creatures with exoskeletons like insects, having fewer than 6 legs means too much loss of stability, because they rely on a tripod of 3 legs while moving the other three.
However, mammals and the vertebrates in general are larger and don't have exoskeletons and can use balancing and moving strategies that insects cannot. Further, they have evolved with four appendages since before they walked on land.
For a 6-legged vertebrate to evolve, there would need to be a selective advantage for having 6 legs versus 4; since vertebrates get along just fine with 4 legs, adding two more isn't likely.
I would also add that just because you can observe mammals with more than 4 legs does not necessarily mean that is a heritable trait. It could instead be caused by environmentally-induced errors in development. I think for the cases you mention this is probably most likely.
Lanham, U. N. (1951). Why do insects have six legs?. Science, 113(2945), 663-663.