I think it is because of the state of decomp poor little Caylee was in by that time.
I have no other way to describe it other than Casey would have taken half of Caylee's face away if she tried to remove it, parts of Caylee's body were probably already getting very messy, I believe she put Caylee in the several bags after she had been in the trunk for a while in that VERY HOT HOT florida sun/weather, Casey had no idea about how the decomp would progress, and the bags were because that is how she had to transport her because of the decomp. The tape may have already started to slide down as part of Caylee's face had been in stages of decomp already. To remove the tape at that time would have been more than even Casey could deal with. Too messy and she wanted to just get rid of her as quickly as possible, Caylee's body was probably difficult to deal with due to decomp.
the smell was probably horrendous and Casey had to smell that smell for quite a while, wonder if she can still smell it in her head at times today?
I feel bad even typing that. MOO, IMO. etc.
I agree with your reason for not removing the tape if the tape was the murder weapon. FCA is intelligent (remember JA told us) but she must have been horrified at the smell and condition of the body in such a short time.
In my opinion the duct tape can only be explained by two reasons.
1. It was the murder weapon.
2. It was not the murder weapon and was placed on Caylee soon after death to make it look like a kidnapping.
Either way, most of us agree that it was murder.
I have flip flopped around but I think it is #2.