Loads of 'real' bike races, and virtually all time trials are run on open roads too. Tri- and duathlons. In fact road closures are pretty damn rare, and only really happen for the most important events.
+1 Even Fred Whitton, arguably the best placed sportive for road closures (due to it's popularity, and dangerous nature of some of the descents) has failed to make any inroads into that particular area.
Most actual races are held on open roads, closing roads in the UK is a major, major undertaking. Surrey Classic this year is a great example of what we're talking about. Thousands of people involved.
Obviously sportives aren't races, but a lot of participants treat them thus, and the standard of riding is often shocking. I guarantee any Sunday group ride is far far better behaved and organised than pretty much any sportive.
My 2 penneth
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