Because they've always done so? It might be because border control is harder (especially with borders in mountains), so they prefer to have weaker border controls, and more controls once already in the country. You are also supposed to register wherever you sleep, but this is done automatically by hotels (and ignored by everybody else).

In Italy (and, I think Switzerland?) the police even visit you apartment, to make sure that it is big enough for the number of people who are supposed to be staying there.