Because these institutions do not exist in Scotland. Once children have sat their National 5s they stay at (the same) school to do their Highers if they are able, more National 5s or National 4s if they are not, or to do a mix of other qualifications. Some will leave to take up an apprenticeship or go to college to take a practical course in something like hairdressing, joinery, building.
But from what I see from friends with kids the same age in England, many schools require minimum grades in GCSEs to win the right to return to school post 16, if they operate a 6th form at all. Do no kids in England not do great in their GCSEs, but return to the same school the next academic year to do maybe one A-level, a couple more GCSEs and an external qualification in catering or something?
So your child leaves school at 16 and heads off to "sixth form college" for their A-levels - how does that work? Is there homework? A uniform? Compulsory attendance? Parents' evenings? Are they in full time, 9-3 monday - friday? Do sixth form colleges take anyone who turns up or do they set entry requirements?
And what happens to the kids who do badly in their GCSEs, aren't allowed to return to school but want another shot at taking them?