There's a number of clashing pressures going on.
Unrealistic government targets turning schools into the middle man. Some bad luck with an illness like flu or chronic illness can hammer the data, especially in the autumn term. Common illnesses are weighted towards the first half of the school year.
Lack of support for SENs. Lack of appropriate school places, lengthy waiting lists, restricted curriculum options and tight budgets. This is a significant driver of persistant absenteeism. A relatively small proportion of persistant absentees has a major impact on whole-school data.
Children can't be "signed off" in the way that adults can. Their access to medication is more restricted than adults (much easier for me to dose up and function than my DCs). Access to primary care for prompt treatment for moderate illness is harder than it once was so illnesses can escalate. (I once couldn't get DC to the GP for a chest infection- litterally had texts saying "we have no appointments, don't ring" and his asthma ended up later escalating to 999 and a week off school)
2020/21 policies did damage home/ school relationships and boundaries on illness. Some people did keep under the weather children off (rather than ill), or preventative absences more than before enabled by increased WFH.
Cost of living has pushed the saving of term time absence vs school holiday peak costs. A week here or there seems pretty trivial compared to 4 + 2 months being kept out of school then repeated 10 day isoloations for a mildly sniffly classmate.
Safeguarding means schools have to be more proactive about monitoring absence.
Illness can be quite a nuanced issue and government policies have turned it into a blanket issue.
I don't appreciate having clearly ill children existing miserably in my classes. Schools are too reluctant to let clearly ill children go home and it's a false economy on spreading illnesses further, but equally it is normal to plough on using over-the-counter remedies for feeling a bit below par.