Why do you think books reflect the period they were written in?

Because that's what the author knows. Even if you write a book about the future, you're going to define the future by what you know today. So every time you do something creative, it will feel like it was made in 2016 because it has 2016 ideas in it. Even if the book is about the past or the future or a made-up place.

A lot of the time they respond to the social norms of the time one way or another.
Even in books that aren't specifically intending to critique the culture they're a product of still respond by having the characters act according to the culture they live in. People in the nineties use pagers for example. It's not intended to be a response to the period, but it is one because it exemplifies the culture somewhat.
And trust me, I know how you feel. I have to write an essay on the function of satire in the 18th century, and there's a lot of stuff to do with how it responds to the culture of the time. So we're in the same boat on this one! :D

Because people generally write only what they know.. especially in the past when less people traveled or had access to books and no one had internet. They only have their surroundings as a frame of reference, our imaginations are usually only as vast as what we have experienced, seen, heard or read. Also they need to appeal to the fashions or trends of the time in order to sell.
I think these rules only apply to the past as now information is so readily available we can write about what we dont know if we research it. It also depends on when the book copy was printed and not just when it was originally published.
A good example of this is Jane Austen.. who wrote about her own society and goes into great detail about the clothes of the time. Every movie adaptation of her stories though changes the style of the clothes slightly to suit the body shapes and fashions of the time they were made.
Another good example is the bible. Every picture version of the bible depicts the fashions people were wearing at the time it was printed.. ie a 12th century bible manuscript has the characters wearing 12th century clothes and a 15th century one has them wearing 15th century clothes.

Because the writer always puts a bit of him/herself in to their work.

I suspect one reason might be language.
It must be easier to write in the language of our time, rather than in any other period in history.
Another reason might be the idea that we write what we know.
So most stories will be relatable to those reading.

Of course. The old Sherlock Holmes stories left me completely puzzled at times due to the choice of words. I remember a reference to someone "ejaculating in the corner" during an argument

Well, if they were written in the 1600s then you'd want to write something that people living in the 1600s can relate to.

A writer has to write from themselves & their own experience, in the end, however much they project into a different era or place.

A lot of good explanations have been offered - I don't think I can add much. Good luck with your assignment.

Language used - references to history or contempory accounts