I think it boils down to human psychology and outward appearances.
Jesus commanded his followers to eat of the bread and drink of the wine. That is the objective behind him passing it to them. He didn't want them to pass it just for the sake of passing it. Passing the emblems is not an end in and of itself. Also, Jesus was not commanded the mere attendance at the event. Attendance is not an end in and of itself. Attendance is only a means toward the ultimate end of partaking. Thus, if one is not going to partake then there is no need to pass the emblems to him. In fact there would be no need to be present at all! But this reality would raise very ugly implications for the JWs.
In most congregations there aren't any anointed JWs. So there is no need to pass the emblems. In fact, there is no need to have the observance at all since the observance is ultimately about partaking of the emblems. So if JWs were to really be truthful and practical about this issue, most congregations should not be having this observance, and for those that do have it, only the anointed should be in attendance. Did Jesus instruct his disciples to make the event a public spectacle inviting all, including non-christians, to be "respectful observers"?
But if JW congregations were to rightly not have the observance (based on JW theology of who is not to partake) that would show up the JWs in a very ugly light. Their disobedience to Jesus would be shown up too starkly to the public. So to resolve this they have to dishonestly shift Jesus' requirement for the event from one of partaking to one of being in attendance. That is why you will hear JWs say, without thinking, that they attend the Lord's evening meal in obedience to Jesus. Really? Did Jesus command his followers to be present? No. He commanded them to partake. Being present is only a means to that end. If you're present but not partaking then you're not obeying Jesus' command. So they have the event to give the impression of obeying Jesus. But even that is not enough.
If they had the event and did not pass the bread and the wine that too would show up their ugliness too starkly. So they go even further by passing the bread and the wine to people they know are not anointed and will not partake! Why? Again it's to give the impression that they're obeying Jesus. "Jesus disciples passed the bread and the wine and so do we. See? We're doing exactly what the first century christians did. We're not strange! We're not a cult! We're passing the bread and the wine in obedience to Jesus, see?"
So it all amounts to this: They (Watchtower) are acutely aware that they're not doing exactly what Jesus commanded and they feel self-conscious about it. They are worried that it shows them up in a bad light. So to combat this they do their best to do as much as possible to produce the outward atmosphere and image of partaking, without actually partaking. This is why they put so much effort effort into having the observance on the right day and at the right time - after sunset - and making sure the bread is without salt and the wine is unfortified, etc. All of this minutiae of legalisms is a psychological way of professing their righteousness and making up for the fact that they're disobeying Jesus' command to partake.
Here's an interesting thing to ponder on: when was the last time you saw, in Watchtower literature, an evening meal scene showing one of the disciples actually in the process of biting of the bread or drinking from the cup? I can't remember ever seeing that. It always shows them passing the emblems but never in the moment of eating it. So Watchtower is shaping the subconscious of JWs to associate the memorial with passing the emblems - not actually partaking.
