... because people think it is. Scarcity has nothing to do with it and I don't think the scarcity argument holds up over the fire regardless. All it takes to mint more coins than everyone thinks is community consensus and if you are looking at a 100 year time horizon and competition between cryptocurrencies, that is not out of the question.
Scarcity is important, because it assures investors and dealers the currency is really decentralized, since the developer will never be able to create new coins besides the 21 million in order to execute market manipulation schemes, like dumping the currency for personal instantly gains.
It's scarcity what also raises trust in bitcoin and its legitimacy. If it wasn't for this characteristic I don't believe bitcoin would reach to the adoption and price levels we have been seeing so far.