I don't understand why migrating humans seems to surprise anyone. With stone-age technology, people can walk across a continent within a lifetime or sail across

I don't understand why migrating humans seems to surprise anyone. With stone-age technology, people can walk across a continent within a lifetime or sail across an ocean (Polynesians). What obstacles would have stopped million-year old pre-humans? Could it be they were travelling all over the world quite often but many of them ended up not leaving any descendants or fossils?