I would argue it's us developers that let the managers do this. They ask us how long we give a number. They say that's too long can you do better. Instead of being insulted, we hum and we haw and we say sure. So the managers learn that the first number we say is wrong and just to keep asking and they get a better number.

If developers gave a estimate (estimates should be ranges) and stuck to it (and delivered on it) then the managers would trust our estimates a bit more.