> Why does there seem to be much more market for NAS than for direct attached external HDD?

I can access my NAS from anywhere in the world, but you can only access your direct-attached drives when sitting at your desk.

I can hide my NAS in a closet, but your direct attach drives are wasting valuable desk space and causing noise in your workspace.

My NAS has a software raid (raidz2) so any two of my drives could die without losing a single bit of data. Technically this is possible with direct attached drives too, but usually people aren't attaching multiple external drives to their computer at the same time.

Multiple people/computers/phones can access my NAS simultaneously, but your direct attach drives are only usable by a single computer at a time.

I can use my NAS from any device/operating system without worrying about filesystem compatibility. With direct attach drives, you need to pick a filesystem that will be supported by the devices you want to plug in to it.

The downside is a NAS is running 24/7 which will consume more electricity than drives you only plug in on-demand, and file transfers will be slower over a network than directly plugged in to your computer, but 99% of the time the speed difference does not matter to me. (It really only impacts me when doing full-disk backup/restore since I'd be transferring hundreds of gigabytes.)