Question

The cyclopropylmethyl carbocation is exceptionally stable, than why does it
undergo rearrangement?

Answer 1

Even though it is stable, there is still exceptional bond strain on ring.

Answer 2

The cyclopropylmethyl carbocation is exceptionally stable bcoz it can undergo ring expansion which will make it a cyclobutyl with less angle strain as in cyclobutyl angles are 90 degrees and it is nearer to 109.5 degree bond which carbon forms(or likes to form).