The new tag requirement used to be lower on Stack Overflow, just like other sites, but has risen as Stack Overflow has matured.
The odds that you need to create a new tag on Stack Overflow are now extremely low. If you find yourself wanting to create a new tag there, you're probably not looking at existing tags hard enough. Re-tagging, on the other hand, is a fairly common exercise.
Contrast that with younger sites, where the tag situation has not settled down yet. New tags will still be more common, and most users won't yet have gained the experience needed to retag correctly.