Voldemort cannot shift here because he does not exist in this reality. You can only shift into a reality where you already exist. Because there is an infinite number of realities, everything is possible. (To quote ‘the fault in our stars’ “there is an infinite number of numbers between 0 and 1, and an even bigger infinite between 0 and 2”). For example, there would be one reality where humans evolved differently and have gills and live underwater. there would be another where humans evolved with wings. Because of this, there are realities that are almost identical (eg. in one I wore a red shirt today and in the other I wore a green one) and there are ones that are completely different (In one I wore the red shirt and in the other earth never existed). Hypothetically if Voldemort were to shift into ‘our’ reality, he would shift to one exactly the same EXCEPT he would already exist there.