|I think there are several reasons to name some|
1) Archives can be huge so scanning them can take a very long time
2) Archives can have passwords on them (not sure if this can prevent a scan?)
3) Some Archives will have other archives within them to attempt to conceal the files, again I am not sure if this is accurate but I have seen this a lot.
others may have other suggestions but immediately this is what I think or have experienced.