Such a provision would be totally inadequate for the type of young person who is in a care placement needing continual two-to-one or three-to-one staffing.
That means two or three members of staff monitoring them 24/7, in addition to any other staff who interact with them 1-2-1 like teachers or tutors etc.
(Not even counting the general staff - cleaners, admin, etc.)
Left alone in a flat, they would likely smash the place up completely, including their own possessions, then self-harm, or go and get drugs, and invite all the local drug takers home.
They would be evicted within days, if not dead.
Of course, when they turn 18, they usually end up in prison or a secure mental hospital within a short space of time.
The idea behind these very expensive placements of a few weeks is that there is a chance to "stabilise" the young person enough to go into a normal care home with a lower staffing ratio, by giving them an intensive intervention.
It is meant to save money in the long term - which it can do if the young person is about 13 or 14.
At 17, it is likely that ship has sailed.