Why doesn’t conventional antivirus work?

If you download all website’s content to your local computer and check it with conventional antivirus installed on your PC, this will lead to no result. First of all, the conventional antivirus finds only certain types of viruses and deletes files instead of curing them, thus deactivating the website. Moreover, the conventional antivirus is unable to detect backdoors, as these are regular executable files similar to all other website’s executable files.