When the raise becomes the shove
The rule
Below about twenty big blinds, raising and folding to a re-raise costs more than moving in would have.
Open to two and a half and fold to a shove and you have lost two and a half blinds with no chance to win the pot. At a hundred blinds that is a rounding error. At twenty it is an eighth of your stack, given away for nothing, and it happens often.
So the raise and the all in collapse into the same action. It removes every decision after it, which is precisely the point: with a stack this size there is no street left where information could have helped you.
Read more: stack depth, expected value