Well, why not? It's simple, it's consistent, it works.
Yeah, I tend to remember them better by number, as well. I can remember most of the titles, but I have to think for a moment, where as if you give me a plot synopsis I can think of the book's number almost instantly.
I think that's probably because only a few of the titles actually had anything to do with what was in the book. #10 The Android, #8 The Alien, sure, those are solid. And, yeah, #3 The Encounter was about an encounter of sorts, and #2 The Visitor involved visiting places, so while they could have been named better, they still make some degree of sense. But, #5 The Predator? #16 The Warning? Not remotely about anything contained within the book.
The worst by far is #24 The Suspicion. Nobody is remotely suspicious of anything throughout the entire book (except maybe very briefly in the first few chapters). The Helmacrons show up, they announce what they are and what they want, and the plot proceeds. They're easily the least suspicious aliens the Animorphs have ever met.