Tesla needs to announce them far in advance so they get funding. They can’t just create another Model, start mass production and delivery without anyone noticing and caring to post it online. So they capitalize on the hype and sell as man as they can.
Furthermore there is no Tesla Model 3 from any other company. There is not even a Model S or X from any company. There are some products that are close but lack some desirable features such as being purely electric or being “techy”. For the Model X there is the Volvo XC 90 plug-in with 19 miles of range which wouldn’t get many people to work in the winter. The screen is a bit Tesla like but from what I have heard is that Tesla’s screen (and infotainment system) is much better.
For the Model 3, the only true competitors are the i3 and Chevy Bolt. The i3 is selling ok but does not have a lot of range and is a small vehicle. For its price, it is not really a good deal unless you are looking for any ev. The Bolt is a much better choice but many do not like its style. It is also not really available because it is often seen as a complicance vehicle. I have heard that people were laughed out of the store when they wanted to buy it. It is also not available in Norway where it is anticipated the most. My guess is that the 2018 Nissan Leaf will be the best choice this year but compared to a Model 3 still a compromise because it lacks (amongst all non Tesla) a proper quick charging network.
So the real question should be: when will the other manufactures start delivering?