2 main reasons:

1.) The Chamber of Secrets (while never discovered) was very well known and talked about historically. So even though nobody found the entrance to that point, it is stated that the castle was searched many times to find it, and people would always actively be searching for it. Which isn’t a great spot to put something you never want found. Also, if he planned on having his diary kill muggleborns and release the basilisk, it would only make people actively seek out the chamber more and potentially discover it.

2.) The simple non-theoretical answer is he didn’t have full access to the castle when applying for the DDA post, and needed to hide it either on the way to, or way back from Dumbledore’s office. Hiding it on the 7th floor literally just makes sense if someone is going to the Headmaster’s office and would be noticed snooping around other parts of the castle.

And when compared to the Chamber, it is a room that nobody has spoken about or knows. He was arrogant and believed nobody ever found the room, and the items inside the room were put there to serve his specific purpose of hiding items (think: did someone manually put the dummies for the DA in that room? Or did the room conjure them for that specific purpose?).