They're big and cheap. My "server" (desktop with delusions of grandeur) has an array of 3 2TB drives (Western Digital WD2003FZEX) running in a ZFS pool. I run Nextcloud and sync all my photos, videos and documents to it. I don't need high speed storage, just a lot of it. Back when I built it (maybe 8-9 years ago) those drives were a reasonable price for that much storage. If I were to re-build it again today, I'd still pick spinning rust over solid state drives. I can get 8TB drives for ~$180. For the same price, I'd get far less space on a solid state drive and the extra speed wouldn't be useful.

Buying the newest tech isn't always the right choice. You should pick parts which fit the use case.