One problem is that it's hard to see how decentralizing work for profit could ever be more efficient than centralizing it. In the decentralizing case you must do the additional work of dividing the task into separate pieces, distributing the task, and validating that your workers are actually producing valid results and not trying to cheat you somehow. In the centralized case, you don't have any of those problems.

Analogously, if I wanted to have a "digital money" I could just have a key value store database running on my home desktop. I could probably handle multiple thousands of transactions per second - thousands of times better throughput than bitcoin, and with only a single machine. The centralized case is just much simpler, faster, and more efficient than decentralizing. I think the sample concept would apply to useful work.