Because there is no reason according to evolutionary theory that we should; the theory makes no such guarantee, and in fact, gives more reason that they should be extinct that not, as any such survivors would have to compete within the same niche as modern humans. They are either extinct from competition with humans or were genetically close enough to humans to meld into part of our species. Our closest living relative is the chimpanzee, which, in a strict genetic sense, is almost the same species as a human, (though certainly not in any intellectual or spiritual sense).
I've answered enough of your questions, if you want more (and better) answers get yourself a few good science textbooks. How about answering my question - what do you think about any one (or more or all) of the points I expressed about the Bible in Post #128? I wouldn't bring it up, but since you've declared that the Bible is the logical funnel for every thought you express, it certainly is central to the issue.
Does it strike you as inconsistent with evolutionary thinking that the chimpanzee can have so much genetic similitude with humans, yet be so obviously distinct? All the talk about evolutionary trees and branches would be much more scientifically sound if there were but some documented (in human language!) cases of those moments where the diverging species was observed.
As far as I know, most species of apes and humans are hearty enough and plentiful enough in population that we ought to see at least one or two examples where the distinction between the two has been difficult. That there is no such case does not prove evolution false, but it certainly is one worth raising for consideration.
As it is, comparative morphology makes for an easy way to support one's fundamental assumptions while sounding scientific at the same time. Hence the story of life is told in any number of ways among evolutionists, just as the story of creation is told in any number of ways by would-be theologians. In either case, science in the strict sense has been abandoned for conjecture, both reasonable and unreasonable.