Because it better matches the historical models we currently have of the early human migrations, as I discussed somewhere else in the forums. It is still not as fast as human where able to migrate in real life, but it is an improvement.
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I can't say yes or no because I don't know. I will certainly improve the migration model in the future, although I'm not sure if that will translate to faster migration speeds for early humans or not. There are many, possibly unknown, variables involved that I have no idea how to model. It would probably take an actual anthropologist to figure out.