It's isn't just branding though, if I'm right. The wake word needs to be something that is easy to pick out from a complex audio environment and not be a regular part of common speech so cause confusion (computer fails a bit in that regard) so allowing completely custom wake words might cause reliability issues. Also some might choose wake words that the brands don't want to be seen listening for: I might choose "slave" as a Blake's 7 reference for instance but that could easily offend some if they overhear, and there are many other epithets, slurs, and swears, that would not be deemed suitable either.