I suppose the answer is that for a World Cup of 32 nations to truly be a World Cup, it can't just have the best 32 nations. It has to have representatives from all around the world. If it was just the best 32 nations, then no doubt Italy would be there... but Italy's gain would be the tournament's loss, because a best 32 World Cup would be mostly European, with a few South American nations, and maybe a couple from elsewhere.
The thing that makes a World Cup so great is that there are teams from everywhere, and a decent number of them, not just one token team from Africa, for example, but many.
And this also makes qualification more exciting. If most European nations qualified, then there'd be little sense of jeopardy in the qualification process. Far better to have limited spaces available.