O'Reilly Author here on a throwaway account.

This is how it works currently due to the volume of manuscripts they are working on at any given time.

My editor told me that you used to be able to ask or try to make a case for a specific animal.

If you are writing a book about something that has an official animal or something, I am sure you can have some sway, but for the long tail of titles, you get one assigned randomly at some point.

If you are planning to publish a book, they are great to work with since they work with more engineer authors than other publishers may have experience with. A big part of it are the editors you work with there.