This is likely due to culture established by the teams working on old, mature products. There, you're going to work with at least 10 people to launch trivial projects. In my case, it's usually 20~30 people with a blast radius of 100~500. You're not going to casually 1:1 with all of them since you and they are all busy. If you don't get a good review from stakeholders, there's a good chance that some angry folks will come for you and make you roll back your launch.
In these contexts, design docs are meant as an asynchronous communication tool for information heavy topics. And this is so asynchronous, you will talk to those people join 10 years later if you product becomes so successful. I've been saved multiple times thanks to some random design doc from 2010 that explains the weird decision that still haunts us. This probably doesn't work very well on lean/small teams or less complex tasks. But engineering culture usually has its own rationale and context, even if it has become a cargo culture.
In these contexts, design docs are meant as an asynchronous communication tool for information heavy topics. And this is so asynchronous, you will talk to those people join 10 years later if you product becomes so successful. I've been saved multiple times thanks to some random design doc from 2010 that explains the weird decision that still haunts us. This probably doesn't work very well on lean/small teams or less complex tasks. But engineering culture usually has its own rationale and context, even if it has become a cargo culture.