It's only partially a question of quantity.

The larger factor is the amount of labor involved in dealing with a bad drive.

Their (large) operation seems automated so I assume if they have a single drive failure in machine #1234 (which has 50+ drives) they have an automated way of switching off said drive. Then they leave it there until the entire rack is replaced many years later.

Other operations have to send a human to manually replace every single failed drive as soon as it fails, which is very expensive as a % of the cost of the bare drive.

They buy enough drives that there's another point of data that's needed for them. Availability of the disks. If they need to expand a few 10s of petabytes they may only be able to get enough disks from one group regardless of reliability concerns.

Thats what surprised me when I learned that they were "shucking" drives from off the shelf external drives at some point.