There are probably a fair amount of code reviews where constructive feedback is needed but not given because the reviewer adhered to the old addage "If you don't have anything nice to say, don't say anything at all."

Tact takes effort and discipline, just like programming. To draw an analogy, maybe it's like when a dev foresees a bug but can't figure a good way to address it (or isn't motivated to address it, or doesn't have the confidence, isn't fully convinced it's really an issue, &c) decides to turn a blind eye and figures it'll get resolved further down the line if it becomes a serious problem.