This is partially because high street shops starting, especially those the three notorious ones - let's call them SuperNova, Tikka and Sons Of Richard - those three are absolute cowboys when it comes to any service, let alone customer service - there is actually written store policy at SuperNova which says that if customer buys any item, let's say two grand worth of plasma, walks out of the store, opens the box and finds that TV set inside is actually pink, or scratched, or has been clearly used before - if he tries to return it managers must do everything in their power to charge said customer 20% restocking fee on grounds of "open box policy" hanging above Customer Service counter. Check it out next time you visit the shop. And don't ever buy anything over £50 from them.
Anyway - I digress - another issue is - high street shops don't do it because knowledge level of their staff is too low to verify if the item is actually what it should be and if it's fully working, so people would keep returning stuff that failed to chip properly or where internals were transferred to another box for upgrade etc etc. It's a known fact students in London would "rent" PC peripherials, printers, and cameras from Argos for years before they started exclusions on their no quibble money back policy.