As this is a tutorial, the snippet will likely be copy-pasted into production code running on God knows what OS by someone who can't imagine why there could be

As this is a tutorial, the snippet will likely be copy-pasted into production code running on God knows what OS by someone who can't imagine why there could be a problem with seeding from the system clock. Using a fresh instance allows the code to be portable, noob-friendly, and concise, which are important traits for its context.