It's a business. That's all.
AFAIK the Canadian Mint doesn't use the country's bullion reserves to make their coins; they buy it in from wherever, and sell it after minting it.
People buy it because "official" stuff carries a greater premium than "generics" because they are more recognisable (ie, an American Eagle, or Canadian Maple Leaf, or British Sovereign are very well known in precious metal circles - you can easily trade them in, should the Global Financial System collapse).
Some people collect stamps, some collect... I don't know, whatever. Some like coins.
Some like just owning silver/gold and will buy the cheapest vs spot - be it bars, ounces, whatever. But others prefer limited edition, 'special' stuff.
But, yeah, it's basically supply and demand - as long as people want to pay $50 for 1Oz of silver (when spot is $20 or whatever), of course the government is happy to sell it to them.
IIRC in Canada the Mint is a separate Crown Corporation - they are there to make money and, er, make money while they are at it :)