(2-part post):
I apologise if this has been covered above but I've not yet read through all of the responses.
Your question asks ". .why can't these days be incorporated into the quarter of a years hoilidays (sic) teachers get each year?" The answer is that they have been!!!!
The school year for pupils (in England and Wales) has always been 190 days. Before the introduction of 'INSET' (In-Service Education and Training) days, teachers also had to attend for 190 days per year (although, of course, most would also have been going into school on several days during the holidays, to prepare for the forthcoming term).
When INSET days were introduced they were ADDED to the teachers year, meaning that teachers now have to attend for 195 days per year.
So your question is founded upon two fallacies. Firstly that the number of INSET days is increasing. (It isn't; there have always been 5 INSET days per year, ever since their introduction). Secondly that they've reduced the number of days that pupils attend school. (They haven't; they've just increased the number of days upon which teachers have to attend).