Because they have to treat everyone the same. You can't end up with a situation where people suck up to school staff and have chats all the time just so they won't be handed a letter that the system just generates automatically.
I work in a primary school office and am involved with monitoring attendance. I roll my eyes at some of the things I'm expected to do by Ofsted and the DfE. It's all for the sake of record-keeping, to show that Ofsted that the school tracks and monitors attendance levels of every pupil, and works with families to support them with low attendance.
What is far too simplistic in my view are the suggested targets for attendance. It's supposed to be done by percentages. But the percentages don't change over the year. This is problematic because if, in the first week of September the child goes home sick at Wednesday lunchtime and is off the rest of the week, by Friday they're already on 50% attendance. Persistent absenteeism is anything below 90% but obviously in this case, the percentage figure means very little looked at on its own. If a child later on in the year was at 50% attendance it would be extremely concerning and would have involved the EWO, parents invited into school for support meetings, possible fines if it was holiday etc etc well before it got to that level.
I can tell you that the letters are generated automatically. While staff will do their absolute best to make sure that no letters are actually given to parents of children who have had long periods of illness in hospital etc, they will still be printed because any absence is still an absence according to the computer system and occasionally admin errors happen where the letter is given in an envelope to the parent by accident.
If it's normal run of the mill illnesses, then some children are just prone to being ill a lot, especially at the younger primary end. Schools do understand this, but have to give you the letter regardless so that you are kept informed of the percentage levels. I guess the government hopes that it may help deter parents from taking their children on holiday to Floriday for 2 weeks in term time etc.
In my school we do hold regular meetings to discuss the children with lower attendance, so that we can get a picture of what's going on. Believe me, there is a wide range of reasons a child is off often from the totally understandable to the downright ridiculous, so it does need discussing. From absences due to a long-standing or newly developing health condition, to a parent with mental health difficulties who finds it difficult to organise their life so that daily attendance at school is a priority. To mollycoddling parents who keep their child off at the slightest runny nose or take their child at face value every time they tell them they have tummy ache (and then once they know they're staying home they have a miraculous recovery). To flaky parents who just don't see school attendance as important. "Can't get there because of the snow" even though they live in the next road type of parents. To the ones whose children are off every Friday (and it turns out they have a caravan in Rhyl).
Often those meetings flag up ways in which we can put support in place to help a parent get their child into school more regularly, so it's important we have them. Often the first time a parent realises that their genuinely ill child has had an unusually high level of illness compared to their peers is when they get an attendance letter from school to flag it up, and prompts them to ask for investigations with their GP. Mostly any blood tests etc for children who are ill a lot throw nothing up and they've just been unlucky. Very occasionally medical investigations throw up something very unexpected and serious.
If YOU know you've genuinely aimed at your child attending school every day that it's open and just been unlucky with illness during a particular half term, and not kept your child off at the drop of a hat (including if you have a hangover or the weather is awful and you don't fancy leaving the house), then the chances are the school will know that too and you don't need to worry. Carry on as you are. The percentage attendance will go up as time goes on if your child DOES attend regularly.
My advice if you have a child that often says they don't feel well and you don't know if they're being genuine (and some are very good at tugging on their parents' heartstrings, changing their voice to a baby voice, big sad puppy eyes), send them in anyway, and school will send them home if they prove to be genuinely ill.