I can't speak for all countries where children start school at 6/7, but in the country I have experience of it is as another poster has said - the children learn much, much faster because they are developmentally ready for formal learning by 6/7.
However they've usually attended a preschool like non comulsory set up from ages 3-6 which has been very much like 3 years of reception and covered all the independence skills and been 3 years of learning through play in small groups, just no reading and writing. Endless social skills, cultural capital (the stories and rituals and fesivals of the culture), endless social skill work, outdoor play, topic based learning abut where food comes from and the seasons and so on. Learning songs and putting on plays. Endless crafts and painting and drawing and construction toy play and puzzles.
Really counting reception as starting school and not counting the 3-6 preschool years in other countries as school is largely nonsense.
You don't get children starting school unable to tie their shoe laces unless they have a recognised relevant special need, much less unable to get changed for PE and put their own cost on.
Once they do start school it's no nonsense, full steam ahead and they are reading and writing by Christmas and expected to be using a fountain pen to write in the equivalent of cursive in year two.
They don't do the enrichment that UK schools do as part of the timetable - things like school plays etc. are extra curricular activities offered free of charge but outside the timetable, and the teaching is fairly old fashioned and fast paced.
Children have to catch up at home if they haven't finished their work in school and differentiation is a fairly new idea and not well done or done much at all - teachers who do offer differentiated work expect to be recognised for going above and beyond, unlike in the UK where it's a basic requirement.
Children as young as 6 often have an hour of homework per day and deciding your child isn't going to do it isn't an option unless you want them to fall behing and not get a recommendation for an academic secondary school after primary.
School is only compulsory for 9 years but A level equivalent means 12-13 years of school (the A level equivalent syllabus is mostly 3 years as they cover more subjects - no dropping maths or foreign languages, though they also specialise somewhat on subjects of choice, some areas have experimented with doing it in 2 years but mostly gone back to 3).