…why never triplets, quadruplets, etc.?
they are according to this site only 2 cases of all three triplets conjoined, more cases with two out of the three conjoined and one normal
IIRC, conjoined twins are always identical twins, never fraternal. Triplets and higher multiple births are usually fraternal, and rarely identical. Identical twins are produced by a single zygote dividing – conjoined twins by incomplete division. Imagine how unlikely it is to have two or more of these events happen at once.
Right. It’s very unusual for a zygote to incompletely divide, which is what causes conjoined twins. The odds of it happening twice in a very short time period are very small.