Hi,
archive files are safe... there is not need to scan them in real time, this will impact the performance especially nowadays that zip files can be really big (not 1.44 MB like in the past).
Only their content can be malicious but you need to unzip them, once you access to any of those unzipped files, the auto-protect scan it. So, if a file is however scanned before it runs, what protection are you missing?
The same for the number of levels, greg12 gave a good explanation.
So, any AV could have a different approach, of course the more scans you want, the more resources you need; Symantec tries to provide the best compromise and, as greg12 wrote, Symantec is satisfied with those settings because the most of its customers are satisfied with them.