The incentive for giving a reasonable spread of grades is that your students have incentive to try harder. Thus you get better students.

The disincentive for giving all A's is that you disincentivize your students from bothering to put in much effort.

Ideally you want to grade hard enough so that even your brightest student has to put in a lot of work to get an A. That can be hard to do in the current climate of grade inflation, but when I grade I at least try to err in that direction.