Why do we omit preposition before 'these days'?

In formal writing or speech, there would be a preposition. "These days" is an abbreviated version of "in these days." A more formal way of saying this would be "Currently" or "In the present period of time." As for why we shorten it, there are several reasons. People abbreviate sentences when speaking to one another. Sometimes this is done out of ignorance of the proper way to phrase a sentence. Other times it may just be to save time when speaking. As time goes on and people hear others use a phrase, they adopt it themselves, and what was once an odd turn of phrase becomes common and sometimes even grammatically correct.