You cannot sort an array in less time than it takes to look at each thing in the array once. If you did not look at a value at all, you have no idea where it goes in the sorted order.

So every sorting algorithm that sorts n things must take time at least n, even in the best case.

But log₂(n) is much smaller than n. The limit as n goes to infinity of n/log₂(n) is infinite. So for every constant c, no matter how large, there is a suitably large n so that c log₂(n) is less than n.