i mean the diplodocus thing seems pretty similar.
but okay. here's the other thing. as the guy says, only certain parts of these creatures seem to fossilize, hence why they only ever get top and bottom halves of certain pokemon.
so they said, well, we have a fossil reviver, let's take a potshot in the dark and hope some of these top and bottom halves match up. Here, 10yrold kid, you look lucky, like some kind of protagonist, choose two of these, stick them together in some way and we'll see what we get. No matter what, after we study the results it's yours to keep.
And as it turns out, none of the pieces match up. the only result is abomination.
Why do the certain body halves not produce fossils? probably the same as why aerodactyl doesn't have its rock spikes except in mega form. If the body part is too similar to just normal ass rock, nobody knows to excavate and restore it too. those parts might not restore at all, the demineralization process would not treat them well.