Op, you have misunderstood what is happening here. You are putting forward a logical argument, but the reason behind this isn't to do with logic, or the fact that involved, educated parents are perfectly capable of filling in the minuscule gap in knowledge from a child missing a couple of weeks of primary school, the reason is that they can't outwardly be seen to differentiate between types of family.
They are using this as a way to attempt to crack down on truancy, an attempt to force into compliance those families where school isn't really seen as something you do unless you have a good reason not to. I don't know enough about it, but presume that those in charge feel that it's more beneficial to do something about truancy, and if they have to annoy a whole sway of parents, whose children would be in school regardless, then so be it.
An irritating part of the whole debate, in my opinion, is that there are a proportion of children, be it due to SEN, or mental health for example, who find attending school a real struggle, and whose parents would really love for their attendance to improve, however this is a much more complex problem to solve.
The holiday issue is relatively easy way for a government to improve overall attendance because a good number of parents self regulate and are natural rule followers.
I know what you mean because where I live you can take a holiday when you like, but it's up to the parents, or the students when older to catch up. If you don't, especially when older you risk not passing the school year, so it's a big incentive.