It is always possible to speculate about good reasons but none are really convincing to me. It's just their policy it sounds like. Nothing prevents someone who flew in in the morning to get the boarding pass for a flight that evening and then leave the airport for business during the day. It would result in many of the same effects described above.

And many airports or airlines have a policy of requiring an in person validation of documents for an international boarding pass. That part makes sense to me. Online boarding passes dont provide any security at all.

And I understand why the OP tried...you never know what the line might look like in the morning so you have to build in more buffer. Get it the evening before and there is less variability and stress in the morning.