I imagine that such managers feel helpless, placed between the people "below" them actually doing the work and the people "above" them that are the ones imposin

I imagine that such managers feel helpless, placed between the people "below" them actually doing the work and the people "above" them that are the ones imposing the broader but equally impossible deadlines for the business. (To be slightly less sympathetic, a manager might feel that their own pay/prospects will improve if they can produce results faster than rationally expected). In order to maintain a feeling of control, or of providing value, the manager falls into the mindset that the one input they have into the process is that they nominally get to set deadlines (for each task of a given scope and level of quality), even if those deadlines are repeatedly missed. The temptation is that they think that the shorter the deadline, the faster the development and the better the result.