Contracts dictate those random release dates
Netflix's release schedule seems erratic and haphazard, with new shows and movies added every few days throughout a given month (here's all the new stuff that came to Netflix for February) but it's not because it wants to keep you on your toes. Rather, it's a byproduct of negotiating terms and conditions with studios -- who may only be willing to license things beginning at a certain date, or for a very specific timeframe.
You can blame yourself
Netflix doesn't want to waste money on content no one watches, so it opts for titles "that deliver the biggest viewership relative to the licensing costs." According to Netflix's former VP of product engineering, the service uses data about members' viewing behavior to predict demand and then seek out the most efficient content. How does Netflix define "efficient content"? Stuff that "achieves the maximum happiness per dollar spent." You know what makes me happy? The O.C. Get on it, people.