This is only part of the answer, but sirens aren't all the same, although they do tend to oscelate between two notes.

Here is a video summarizing some European sirens and the different ways they sound. https://www.youtube.com/watch?v=J-WDEEorxMc

The reason for the simplicity of the sound is historically mechanical. You can find explanations for how sirens work, but I'm not the right person to explain.