Nature of the beast. It's a trade outlet.

Like all trade outlets, part of the reason they're cheaper is because
they don't tie up staff time on customers making low cost purchases.

Argos is the nearest to a retail business using a trade business
model. You don't get to fiddle with goods or get advice on them.
Customers don't have the opportunity to mix up or damage stock, tear
open boxes or shoplift stuff. Consequently Argos can be more price
competitive.