I think there could be many reasons for it
*Historically it seemed that babies in Western societies always went to bed early so when Dad (white collar worker) came home from work he only had to put up with them an hour or so, then it was adult time for the parents (at least that's what my Mum got told by her Dr when she raised us in the late 60's & 70's)
* Parents getting up to go to work need children up 7am (give or take) so it tends to be what works
* 7ish to 7is works for a mid day nap for older children
*someone's research somewhere probably said that 7-7 kind of day was good for their health - but loads of research has been found to be not quite a good example of fact.
* Also the more people doing what you are doing makes you feel more confident you are on the right path, so it's a bit like follow the leader, 7ish to 7ish works for many in terms of their daily routine so therefore it kind of becomes a "gold standard"
Since no 2 children are the same, the reality is, if it works for your child then it's "right"