I have a couple of thoughts on this, as the parent of four kids (5-13). First, a link with a fascinating map:
How children lost the right to roam in four generations | Mail Online
Second, I think part of the reason that parents are more protective is that the death of a child is so much rarer than at any other time in history. Now, the loss of a child is an uncommon tragedy, whereas 100 years ago, it was almost commonplace. People invest more time and energy into each child nowadays, and no one expects to lose a child to anything.
For our family, we're trying pretty hard to be less protective, and allow ourselves to allow our children to do more dangerous things, and to be more responsible for themselves. I absolutely agree that it helps them to become happier and more mature adults. On the other hand, if anything were to happen to one of our children, I know we'd blame ourselves and probably become far more protective, logic be damned.