In fact, there are dozens of different pre-human "homo" & other species fossils, from Ardipithecus ramidus through to Neanderthals, Denisovans and Floresiensis "hobbits".
Some of these, as you say, do indeed have only one or two samples so far found.
But others (i.e., Neanderthals, Ergaster) have dozens or hundreds of individuals found.
These allow reconstruction of complete skeletons, and from them presumed outward appearances.
Point is: there's more than just guess-work behind those reconstructions.
Boogieman: "Some scientists come up with a conceptual idea of what that primate must have looked like, based on their preexisting bias that the primate must be an intermediate form between other primates and homos."
The bones themselves tell a lot about what those individuals looked like -- no need to exaggerate either their human or non-human aspects.
An honest representation, to the best of our knowledge, is all that can be expected.
Boogieman: "Then they show you a sequence of such conceptual renderings and call it 'objective'."
Hominid skulls, from chimpanzee to modern homo sapiens: