Because this will increase the load of teams too much. They won't have much time to gather themselves up and focus on next race week because they will have a very tight schedule. Only the summer break still wouldn't be enough for them to adapt to this tempo. We all would like to see races every week to be honest but it would be really selfish to ask for this from teams. I think the current formula is much better for spectators as well. Because we are enjoying the season for many months and it leaves good memories in our minds. Otherwise the season would end so quick that there wouldn't be even much time to assess the races.
I really understand that point of view. Personally (probably I am wrong) I compare that situation to other sports, like cycling. Then, for example, you have spring full of one-day races. Eveyrbody knows about the 3 big Tours (Italy, France, Spain), but there are others, many many races. And in fact cyclists very often do not train (a lot) in the meantime - winter is a period for preparations, then having many races only give time for rest - and next weekend there is another race. But it is tru that comparison like that is wrong - many cyclist prepare themself for one specific period in year and it would be strange to hear from F1 team "we focus on races in Asia".
My statement was because it is getting cold, autumn is coming and for me it is somehoe the end of season. October, November - I cannot focus on that. Races on 20th of November - in my personal opinion it is very very late.