Why is a 19-inch cabinet about 600mm wide?

Because the 19 inches refers to the equipment mounting format, not the outside of the enclosure.

A 19-inch rack-mounted product is roughly 483mm wide across its mounting ears. The cabinet then needs space outside that for the mounting structure, side panels, door mechanism, hinges, locks and useful internal clearance.

So a cabinet roughly 600mm wide containing a 19-inch rack isn’t an unfortunate encounter with metric conversion. The additional width is where the rest of the cabinet lives.