One possible reason: it makes little sense for 1 << 33 to be less than 1 << 31 but not zero (assuming 32-bit ints -- replace 31 and 33 with 63 and 65 for 64-bit ints). In other words, if you keep shifting a number to the left, it should keep growing, until all the bits are gone.

Another possible reason: CPUs have instructions for implementing << as it now stands.