I think the average person not involved in education doesn't really understand how teaching works.
For example, say that Year 2 are doing topic work based on science. They are learning about electrical circuits. If the child is absent, they will miss the work.
From the parents' point of view it's not a problem. They can teach their child about electrical circuits and they won't have missed that part of their education.
From a teaching point of view, the teacher has a set framework for the children to present their initial knowledge, their predictions, their observations and their conclusions based on a controlled experiment in the classroom.
The reason that the teaching is done like this is because the teacher needs to provide evidence of the child's learning. The headteacher will want to see this work from time to time to assess that the progress being made is in line with the teacher's assessment. Children's work can be selected at random.
The headteacher needs to do this because they are answerable to Ofsted so they need to be able to produce evidence to support their findings.
If children miss this it is often not practicable to repeat the lessons. What would the rest of the class do whilst the teacher is re-teaching? There are also lots of other assessments taking place throughout the year but especially just before the children break up for half term or end of term, because this is a good time to measure progress.
Parents think the last week of term is a good time to take the children out because they won't be missing much but it does cause a problem for many teachers and headteachers.
All of this is government led. On top of this, headteachers are under pressure from government to crack down on absence. Absence figures are factored into Ofsted 'scoring' of schools so no matter how good the teaching is, high absence will give the school a lower grade. If schools become undersubscribed they will get less funding from the government so obviously headteachers want to avoid this at all costs.
Schools, caught between what the government wants and what parents want are just trying to do what is in the best interests of the children.