The second reason is that animals pre-dating the timeline of man or even earlier in our history are generally much, much, much bigger, more powerful and with extreme appendages (like saber-tooth fangs, 16 ft ostriches, etc). All of our companions are miniaturized, but they all maintain their "natural" abilities like biting and clawing. It would be very hard in battle to have to think about an extinct animal and their movement considering we don't have actual footage of many of these creatures.
It's obviously a complex issue, but we have mythical companions (boggarts and rougrou) which can take on the form of any mythical creature with a wiki page! Which I think is pretty open to be creative.
Normal companions are supposed to be commonplace/normal and we have to keep in mind our current frame of reference. That's why we drew the line there with normal companions.