There's a basic communication problem IME. When I've been in this position (an impossible deadline) it's been because some Sr VP or C-suite type decides that something HAS to be done by a certain date. Like, they want to unveil a new product/feature at CES or some other trade conference so the date is fixed. So this filters down through 3-4 layers of management to you, the dev lead who actually has to build the thing. You take one look at it and it is basically impossible and you can list the 17 reasons why. BUT the problem is that the person that needs to be convinced is the Sr VP/C-suite type. So your analysis has to go back up, filtered through 3-4 layers of managers, each less technical than the last. And of course nobody in that chain wants to disappoint their boss so they hedge/soft-pedal your analysis at every layer. Now the SVP/C-suite type, who typically does in fact have good reasons for wanting this project done by whatever deadline they set originally, only gets a watered-down, possibly incoherent version of you analysis of why the project is impossible by the given deadline. Not surprisingly, they don't find it convincing.
tl;dr; everything in a large corporation is a game of telephone where everyone is incentivized to muddle the message. So any deadline/commitment handed through multiple levels of management is really difficult to negotiate. And the more unrealistic it is, the harder it is the negotiate. Nobody wants to tell their boss that the thing they want done in 3 months will actually take 2 years.