What's wrong? It hard-codes the assumption that one request equals one process, and thus makes it doubly hard to move on to a solution where multiple requests can be served from a single process.

Whatsmore, it limits your objects and functions to only being used in CGI scripts. Some of these objects may be generically useful, for example, useful in cron jobs that perform various routine maintenance related to the website.

Even if you ignore all that, there are valid reasons to make sure that sessions are passed around from function to function. For example, you may wish to allow the admin user, once they're logged in, to take over any other user's session - to help diagnose website problems for example. Doing that sort of thing is much easier if the session is not a singleton.