There's a fair number of kids who aren't ready mentally, emotionally and in terms of maturity to go away to college at 18.
Eighteen isn't the same now as it was 40 years ago either. Certainly not the same as it was 70 years ago.
I think it really depends on the kid. For sure, for some, getting another year before college might be a benefit in getting more mature, emotionally ready, etc. But, for others, it's probably a good excuse to spend another year goofing around and not growing up. Sometimes it's better to be thrown into the lake and being forced to swim.