Because it offers a non-zero chance of the best outcome: a low cost, high benefit outcome where the world changes to fall in line with your expectations. If you do not say anything then the world has little chance of changing. It is a rational result of cost benefit analysis. (Consider that moving to another service is costly, and building a new one more costly still.)

Additionally, I would point out that complaining about things that we don't like is a universal human trait, which you yourself demonstrate. :)