In those days, a book was worth its weight in silver, if not in gold.
(This is a sentence from some grammarbook.)

When I started a new sentence with "In these days~," by analogy, I was told that I should say "These days~," without "In."

I wonder why. I'd like to know if there's some explanation.

Thank you, in advance, for any comment.
Unfortunately, there is no symmetry in these expressions. There is no explanation except that native speakers tend to say these expressions using these words. Emotion: sad (The same is true of many expressions in many of the world's languages. You are sure to find many more such inconsistencies as you continue your studies!)

Teachers: We supply a list of EFL job vacancies

Thank you, CJ, for your reply, now I can stop wondering, and I feel much better.

Emotion: smile

These days is nearly the same as nowadays but it's not used with "in". This phrase is used in the present situation and can be put at the end or the beginning of a sentence. E.g. Kids are really spoiled these days compared to the time I was growing up. In those days, we had practically nothing in the way of toys and luxury. Nowadays, parents give their children Gameboy for birthday presents.