Yes, I'm going to say 'Money' as well. Bully for you if cost isn't a factor but it is for most of us.
I recall someone posting here a while ago on this subject. They were from the UK but had emigrated to NZ. They wanted to take their DC out of school for a family holiday and wrote a long, explanatory (begging) letter to the Head. He replied 'I don't know why you're asking my permission- they're your children'...
Once again, we Brits demonstrate our complete inability to recognise 'shades of grey'. One parent thrashes a poor DC to within an inch of its life. So they attempt to enact laws that make the tap across the legs of dangerously badly behaving 3 year old an imprisonable offence.
Certain parents couldn't give a fig about their DC's attendance at school at all. Ever. So the rest of us get saddled with the laws and fines that make it a serious offence.
Of course, I think there should be limits- that's 'common sense' (remember that?). One should 'get permission' from the head if only to ensure no vital tests are being missed, and there should be a limit on the maximum allowable time (so as not to disadvantage the other DCs whilst a holiday-ee (!) 'catches up') but I believe a parent should be able to expect a no questions asked agreement if the DC is normally a good attender and progressing well at the school.
Finally, I too am a bit alarmed at the amount of time I've seen wasted in school- on an Open Afternoon I saw my DS1's NQT spend TEN MINUTES of an art lesson getting ALL the DCs to search for a pritt glue stick cap. If that's what they do whilst being observed, how much arsing about goes on otherwise??! Yet if WE want to 'waste our DC's time' by taking them OUT of school on holiday, boy are we castigated!