Because the first day back after time off ill or in hospital is never going to be the day to get the best out of a child.
To be honest I don't even like tests on the first day back after. Half term, etc for ANY child. Half term is there for a reason - to give them a break. Not to,spend the whole time working and studying.
Obviously this is even more so if a child is ill or injured. If you're in pain or discomfort, whilst you have more time, your body is also working overtime to repair and heal the body. To,add additional stresses to that isn't ideal.
If a school wants to get the best from a child - which, after all, is what they should be aiming for - they should leave it a few days at least. Most people, adults and children, routinely return to work and school slightly earlier than full recovery. I've yet to meet people who stay off until they are 100T recovered.
As a result: Give the child space to return, get their head round being back in the school routine, get back into the whole routine of studying, completing work, etc. And also to catch up with friends and get that out of their system a bit. A;so, a return after time off can be quite tiring, mentally and physically.
And then, after a few days (week later perhaps), when the child is back into the swing of things, has had some time with less pain/discomfort and is used to being back - that's when they're generally more likely to be ready for assessment and when they're more likely to be up to showing their true potential.
I've yet to meet anyone who performs at their best on their first day back at work or school after prolonged time off.