I think it's just that the business of moving large quantities of people around the place with any degree of accuracy demands that airlines think up a fairly foolproof system and then make all the passengers fit into it, without exception.
Take one part of that system: checking BPs at the gate. Computers now check BPs at most gates... humans, even well-paid and intelligent ones who care about their employer make mistakes. Now consider how many mistakes will be made by ground staff at an outstation who are employed through several layers of subcontractors and not paid very much. Achieving 99% process accuracy is easy enough - even 99.9% is a reasonable human success rate. But if you think 99.9% is good, for an airline the size of Emirates that means putting 86 people per day on the wrong plane.
The error rate might be small, but consider the hassle of fixing those mistakes if a passenger has been flown to the wrong destination (possibly without a visa and hence incurring fines for the carrier).
Some of us have seen airline employees notice a passenger (usually one of us) who doesn't fit the system well, and that employee circumvents the system to help us. The sad reality is that airlines don't want this happening, because if you let people use initiative then a minority will make mistakes that cost money to fix.
There are other reasons too, like a cabin crew change where the new crew don't arrive before the old crew leave - who would evacuate you in event of emergency? Who would notice you ducking into the lav, putting on a reflective vest, then exiting the aircraft via a rear stairs used for cleaners and wandering around the ramp?