They aren't. Macs are just targeted less in the wild than Windows is.
On occasions they have competitions to see who can hack what the fastest for cash and prizes.
Macs are usually the first to fall from the ones Ive seen.
Norton scanned less files because it flags certain "known good" or "trusted" files and does not rescan them unless:
The file changed or its doing a full system scan.
A few other AVs have a similar behavior.
I agree with the others however, find one you like and run just the one active scanner.
As well as Malwarebytes as a secondary on demand.
MSE is probably the best free alternative out atm.