You're right, they should know better, but I think a lot of them have gotten away with it because most of them are not expected to produce written material setting out missing assumptions etc. and breaking down the task into more detail before proceeding to work, so a lot have never gotten the practice.

Once people have had the experience of being a lead and having to pass tasks to other developers a few times, most seem to develop this skill at least to a basic level, but even then it's often informal and they don't get enough practice documenting the details in one go, say by improving a ticket.