ah yes. Because there is a direct and clear road ahead from requirement to implementation. Requirements don't change and if they do it doesn't have any impact on existing parts of the software. /s
An assertive dev will look at the requirements, try to figure out how that fits, and then reply back to stakeholders if it's unfeasible. Everyone else will sit there for a week bashing their brains out against a keyboard and have continually increased anxiety because they've been assigned an impossible task. The time for a dev to be involved in the process is during requirements gathering as a partner to the process, not after a contract for work has been signed by the client.