Two reasons:
The specific US version was an attempt to bootstrap acceptance of the $1 coin-- if you could get a box of 500 shipped to your door, you'd spend them and eventually they'd displace $1 notes. When they went straight back to the bank, no circulation occurred, and they were burning merchant fees and shipping costs for nothing.
Some other countries used "selling coins at face value" as an explicit financial hack based on seignorage (the value of a coin in excess of its buillion content).
Canada used to sell commemorative $20 (and then on to $50, 100, and possibly $200) coins with modest silver content based on this model.
If someone pays $20 for $10 worth of silver, the government profits $10-- as long as the coin disappears into a collector's binder and never enters circulation.
So it worked fine as long as collectors bought them. When people bought them to generate card points, and immediately took them to the bank to exchange for more conventional currency, the plan fell apart, and Canada abandoned the programme.
The UK flavour had a particularly ugly endgame-- the mint basically told banks not to accept them as deposits (https://www.thisismoney.co.uk/money/news/article-3390519/I-b...) which did nothing positive for the programme's long-term survival.
It's a shame these programmes were sabotaged and died. The idea of "instead of stuffing a banknote or gift card in an envelope, I can give you a pretty collectible and feel confident that if one day you need the money bad enough, you can still spend it" was a nice idea while it lasted.