why is safari not updated automatically on my mac

The way Safari is updated is by upgrading your computer's software. An upgrade is when you install a newer version, such as upgrading from Catalina to Big Sur. (An update is to install a newer version of the same operating system.) The reason for this is because Safari is installed as part of the operating system. It cannot normally be individually installed. Incidentally, a newer version of Safari does not always work with every web site. Sometimes it is necessary to install another browser such as Firefox or Chrome to use with certain web sites. They are individually installed and upgraded.

If you are running macOS Catalina or Big Sur, Safari may also be updated from a Security Update (when available) presented via Software Update. This will appear as an Other hyperlink beneath a System upgrade entry.

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