I agree, but there are many reasons for the police to believe others were involved even if no evidence left behind has lead to subsequent arrests.
Here's a little experiment you can try with a few (4) trusted friends. Buy a dozen rolls of duct tape. One person gets to be Dint the others play the roles of the victims.
Forget about not being seen on camera, being seen or heard by neighbors,two story house etc. simply see what it takes to corral and bind with tape each " victim" in a chair. See how many hands it takes, how long it takes, how do you unbend someone quickly if you need to, etc. how do you bind the last person? Once you figure it out you will see many pitfalls and problems involved n just this one aspect of the crime.
Each act that made this crime happen is not as simple as some would have you believe.