This is because USB drive hardware isn't as robust as larger SSDs and HDDs. They're also affordable, usually costing $10 or less for a few GBs, compared to $60 or more for NAS-rated hard drives.
USB drives suffer from similar issues to standard SSDs and don't offer as many capacities as hard drives. A NAS HDD can be bought with a capacity of up to 24TB. SSDs are slowly catching up with ...
There are also those little USB memory sticks you can use ... Of course, SSDs aren’t as delicate as hard drives that normally inhabit NAS boxes, so a tumble likely wouldn’t be catastrophic.